How many times have you gotten to that spot on the route where you'd like to pick up your right hand, but couldn't? If you did, your body would swing out off of the cliff like a door swinging open, your left hand and foot acting as a huge hinge. The problem is that your center of mass wants to reach the lowest possible point it can. Swinging out to hang from the remaining foot and hand achieves just this. How do you prevent it?
I've tried my chest to the cliff, and as shown in the adds you see on TV, it will hold. The problem with this method is that it tends to impede upward progress. Some people like to dyno or deadpoint from such a stance. The inherent problems are wasted energy, and the fact that you are relinquishing control of the holds. The body often swings out and off of the holds (in the case that you do catch the target) burning you up while you are attempting to reset yourself. Other possible solutions to the dreaded "barn door effect" are flagging and counterbalancing.
DOG-SCRATCHING: in this case you might have the left hand down, and are reaching with the right to a higher hold. Uh-oh!!! if you let go with that right hand, you will play barndoor, and your right foot will slide off of the hold. Maybe the foot work needs to change. Try putting more weight on the right foot and pulling the hold in with it. (those of you with a dog will understand this next illustration...) Think of when your dog tries to play tough and bark and scratch, throwing up tufts of dirt behind him by digging in his feet and pushing back. Now, YOU do that to the hold. The inwards force from the foot may help hold you into the cliff, and keep you from swinging off.
FLAGGING: OK, you've Tried dog scratching. It didn't work. You may have gotten so "into it" that you were even barking, and your partner has tears welling up in his eyes because he's laughing at you so hard. Well, before you end up off-belay, you better quit and try another approach. Try "FLAGGING." Once again, you are worried about that right side swinging out away from the cliff. The right foot isn't going to hold. Maybe it's on a smear or something. Try this: Switch the right foot onto the left foothold, and extent the left foot out further left. If there is a hold there, great, use it, but if not, that's OK. Now you are standing with the left hand up above the right foot, with the left leg sticking out to the left. You are essentially in a diagonal position. The body might or might not swing now, give it a shot. If the body still "barn door", try PUSHING (that's right, I said PUSH) off the the wall with the left toe. This torque can be used to counteract the tendency of the body to spin off to the left. You are now FLAGGING.
COUNTERBALANCE DROP: Let's say for a minute that you can't switch feet, or that it still doesn't work easily enough by the "SCRATCH" or "FLAG." Time for something new. Try this. Drop the right foot off of the hold it is on, and cross the right leg back behind the left. This will change your center of mass. You might not have a tendency to spin anymore. This move is extremely EASY to execute, so I often go for it when my C.O.G. is just slightly off. You are using your leg like a cat might use it's tail, for a ballast to improve balance.
COUNTERBALANCE PUSH: OK, The other methods aren't working. You still haven't made the move, and you are getting pumped. It's time to try something that looks funky enough to impress your belayer. (*This is a good idea, since he's still laughing at you for barking and scratching, and you want to shut him up.*) Try a COUNTERBALANCE PUSH. It's a lot like the COUNTER- BALANCE DROP but you will use that right leg for more than a ballast. The right leg is straiten and set as far out TO YOUR _*LEFT*_ as is comfortable, then used to PUSH off of the wall. This is like a flag, where you use this torque to counteract the tendency to barn door off of the cliff. I wish I could post a photo, but I can't. Al Black's "Waddington Gallery" will contain a photo of this in a few weeks, as soon as I mail it to him.
-T