I have seen over the last few years that finger, hand, & Fist Jamming are not exactly an innate technique. Even the books I've seen are kinda vague. This is an attempt at elaborating upon the base knowledge you probably all already have.
As a lot of people will tell you, a good jam can turn the most pumpy position into a hanging rest to place gear out of or recover from. More than once I rested or placed gear from stances that the face hold would have pumped me out.
There are about as many ways to jam your hands into a crack as there are ways to position your feet, and none of them will always be right. Different problems call for different solutions, and this is meant to be a set of suggestions based on my experience, not as instruction nor as a last word on the matter. For lack of 'common names' for some jams I'll be naming the jams after describing them. A few categories I will be referring to will be "Pinkey in" and "Thumbs down." Pinkey in refers to when the hand is positioned like a blade with the thumb pointing at your nose and the pinkey-side of the hand going away from your face. Imagine a kid trying to 'karate-chop' something. For later reference, when the hand is placed in a crack in this position, I will call it "pinkey in" jamming. Certain cases call for another position, "Thumbs down" as I call it. To see this position, extend your right hand as if you were going to offer an handshake. Rotate the and 180 degrees counter- clockwise such that the thumb points downward and the elbow points outward. When the hand is put in the crack in that position, a "thumbs-down" jam can be achieved.
For many of the techniques described, esp. for the beginner, it would be wise to tape up well before practicing/trying these moves. You will be using some tender skin that might not be used to the abuse. Some pain or bleeding could result on sharp rock. Tape mitigates those problems. Make sure that the tape- job is nonrestrictive, yet will not slip around on your hands. Yosemite-style "Tape Gloves" beat the heck out of trying to simply wrap the hands up. Tincture might also help. A friendly trad or John Long Climbing book can demonstrate this for you.
I'll discuss the thumbs-down-jam (hereafter called tdj) first. It is a more complex position, but there are fewer variations of it that I use, so it's more simple. The tdj relies upon the natural torque of the hand naturally wanting to rotate back to a thumb-up position to lock into the crack. This natural torque can be added to by willfully twisting the hand harder into the crack by using your muscle power. While the tdj is best when placed overhead and in a construction it can be used in a featureless desert crack too, the friction between the area of the hand that are in solid contact with the rock. The exact area that will be in contact varies with the size of your hand, the size and contour of the crack, and the height above you at which the jam is placed. Generally it will be an area on the back of the hand nearest the first finger and thumb as well as an area on the palm nearest the pinkey, on the fleshy pulpy part of the hand near the wrist. In larger cracks the hand will come back more clockwise once jammed towards a normal position.
In a tight crack the hand may be so jammed in such that almost no twist is possible and nearly the entire area of both the palm and back of the hand is in contact. Since twisting is unlikely in such a hand placement other methods of securing the hand are frequently required. These actively expand the hand in other ways. The first way of doing this is quite simple, but not usually the best for me. The hand, once in place, can be flexed as if you were trying to cup it to hold water. The fingertips and base of the thumb will be forced outward and make stronger contact with the rock, allowing more opposing force, and thus, more friction to hold you up. A more common variation of this is to make an effort to touch the tip of the thumb to the connection of the pinkey and palm; essentially you are trying to fold the entire thumb, base and all, across the palm of the hand. Although little movement will likely be achieved, the forces between the rock and hand may change dramatically. This technique doesn't require much, if any, effort from your forearms, which can be rested while in said position, saving those muscles for when you call upon them later.
There are a few special notes about tdj-ing. First of all, the hand doesn't have to be placed fully into the crack, it might not fit. The more you get in, the better it will likely be, but it's just like face climbing, not all jams/holds are going to be ideal. Secondly, the jam is often best when placed just above a 'bottleneck' (A.K.A. neck-down) which can't be passed or just as well (if not better) in a slight "punch bowl" in the crack, where it widens on both sides for a bit. The effective neck-down created at the bottom of the punch bowl will help hold the jam. Lastly, if the jam is placed overhead and you step up high, advancing your vertical progress greatly without adjusting the jam, you may get quite literally stuck. A simple demonstration of this can be achieved by putting your hand in a tdj position high above your head. Keeping the angle at which your fingers point at constant, in all directions (twist and slope) bring the hand slowly downward. Notice how it gets more and more stressful on the wrist and elbow to hold that constant position as the hand comes down in reference to the torso. The beauty of the jam is that it locks the body skeletally, not muscularly. The problem is that you can, in fact, get STUCK, and have to back down somewhat to unset the jam. Be aware that you should pay attention to your body. Do not do jams so severe that it induces joint pain. Lastly, get used to a bit of pain in the skin. A hard jam can scratch, bruise, etc etc etc you... Just like anything else, there are degrees. A 5.6 crack won't hurt an experienced. jammer, and a 5.12 might cause pain in the worlds best jammers.
The second common hand-jam is the Pinkey-In_Jam (pij). As described before, this is when the hand is placed in the crack in such a way that an extended thumb points at your body. Although this jam is more straightforward in anatomical position, there are many more tricks and variations I use to get it to stick more ideally, depending upon the placement. The most straight forward way to do a pij is to place the hand in the crack and slide it down to a construction that it can not fit past. Essentially it is now like a stopper placement. If no such neck-down is available, secure, or comfortable, a few other variations can be used. These are much like the tricks used to tighten the tdj. Try cupping the hand or folding the thumb over the palm again; see what works for you. For me, the most powerful of these is folding over the thumb while, at the same time, cupping the hand as if to bring the base of the pinkey and first finger together, as if my hand were being squeezed. While doing this, don't make much effort towards bending the fingers. Let the palm do all the work, you might need those fingers later.
A very different approach to the pij is often used in shallow and/or flaring cracks. Nichole, my partner on Friday The 13th (at Vedauwoo) said this was instrumental to her in her first ever hard/sustained (and shallow) hand crack of the first pitch. (Once again, she was an experienced climber. and capable face climber, but had simply not done any hard jam-cracks.) If the crack is too shallow to accept the whole hand, try putting in the hand in pij position, as many fingers deep as is comfortable.
In some climbs (like Friday the 13th at times) you may only get in up to the pinkey or ring finger, leaving most of the hand exposed, fingers pointing skyward. In this instance, the jam may be tough, as you flex the palm in order to stiffen and expand the muscle running the height of the palm from the wrist to the base of the pinkey. At the same time, bend the fingers that you've gotten into the crack as far forward as is possible while leaving them relatively straighten (bend the fingers only at the base knuckle), so as to put more force on the walls with them. Although this may not always be a great jam, it CAN be very good. The technique can force both the connection between the palm and wrist (which as you can see is not flat) to catch well on any irregularities in the crack while the expanding palm and pressing fingers hold you in. It's possible to hang single- handedly off of any of the described jams if they are in a solid placement.
For both of the jams mentioned, as well as all others I'm going to cover in the posts to follow, keep in mind that, as in face climbing, not all holds/jams face upward. I'd give a rough estimate that of all the jams I do, that 30% or so are what I call "Undercling-Jams." There are most common in a variant position of the pij. To undercling jam the hand can be placed either back behind or beneath a construction than brought outward or upward into position. This can help in all of the same ways that an undercling is used. It enables you to hold yourself in on a jam below shoulder level, to lean back, rest, or high-step between jams as far as 5 feet apart. In the case that this type of jam is done, you generally put in the hand as if you were extending for a handshake, then slid upward or outward. In other words, the fingers might point in or even DOWN as opposed to upwards, but the wrist is not rotated like in the tdj, the thumb still points upward.
As Jens and Nichole (some climbing partners of mine) told me: "You just don't trust them because you are not used to it and they don't seem like they should stick, but they do!" Well, the only way to see what is and is not going to hold you is to test the limits of jamming. The only way to see where the top is is to go over it. Test jams that you think will not hold. Practice and refine your technique. You might surprise yourself.
-T