In reply to DubyaJamesDubya:
Perhaps there is some vagueness about different possible roles.
The outer leg in heel-toe position is surely the critical element in moving up. The inner leg rarely contributes to moving up, but is essential, especially the steeper the crack gets, to holding the climber in, in combination with arm bars or chicken wings and palm pressure.
Sometimes the inner leg is cammed knee against either the heel or outside edge of inner foot, sometimes it heel-toes too, but on steeper things, if possible, it may be inserted high---as high as waist level---and twisted so as to get a heel-toe cam deep inside the crack. If the crack overhangs, then in order to move up, the holding position of the inner leg has to be relaxed, at which point only the arms keep the climber from falling out, the heel-toe cam supplying upward propulsion but little or no effective inward force component. In my not-extensive experience, this is when you fall out.
Maybe it is just me, but if you can't get your hips into the crack, then chicken wings are good for resting but have to be switched over to arm bars for moving up. On the other hand, if you can get your hips in (so really a tight squeeze chimney), then you move up on chicken wings by pivoting the hips up in the direction of the chicken-wing arm, palming low with the other arm to help force the pivoting. The motion is pivot hips up and lock lower torso, straighten upper torso and reset chicken wing, etc. It helps a lot if you have shoulders flexible enough (and strong enough) to have the chicken wing elbow pointed up and not just in.