There are two types of jamming. With the classic "hand jam", the hand is placed in an appropriately-sized crack and the muscles expand the hand inside the crack. The various counter-pressures result in a locked or jammed hand, which can be very secure when properly placed. The second method involves torquing and camming the appendage in a bottleneck or constriction in the crack. In wide cracks, the limbs are often twisted or stacked and in very wide cracks you'll find that wedging and cross-pressures are the only way up.
FINGER CRACKS
Finger cracks vary in width from shallow seams which you can only get your pinky into, to cracks which swallows your fingers up to the third knuckle. Halfway between pure crack climbing and pure face climbing, thin cracks require techniques from both forms. Consequently, finger cracks are often technically demanding.
HAND CRACKS
Most climbers think hand cracks are the last word in crack climbing. The crack is perfectly suited for both hands and feet; the technique is readily learned and very secure and vertical, even overhanging, hand cracks are often reasonable.
CHIMNEYS
When cracks widen enough so that they are easily entered they may lose some degree of security, though they are usually much easier to climb. Knee chimneys are just that: bridging is done between knees and back, variations in size being accommodated by flaring the knees to varying degrees.
LIEBACKING
The basic motion is: pulling with your hands and pushing with your feet, one in oppostion to the other. A typical lieback will find your side flush against the wall. Your hands are clasping the near edge of the crack, and you are liebacking off them.
Your hand are pulling directly out and your feet are pushing the opposite way, directly against the face or far wall. Since the technique is strenuous try to keep the arms straight, which again lets the skeleton, rather than the muscles, absord at least some of the strain. Many times it is possible to leap-frog the hands, one over the other; but often the top hand will remain on the top, shuffling the lower hand up to it, then reaching above with that top hand.
STEMMING
Stemming or "bridging", normally occurs in right-angled or oblique corners. In its pure form, the feet are pasted on opposite walls, perhaps frictioning, maybe smearing, but counter-pressuring against themselves and/or the hands. Essentially, your body is like a spring lodged between two sloping walls. Moving the spring up, which is the hard part, for this requires the tension to go lax, at least momentarily. Usually, it's the diagonal pressure of one hand and one foot that keeps the climber in place and the climber alternates this bridge in moving up.
