The hand wedge tries to split the crack apart. This force is applied directly through the skin to the rock of the crack. Usually, the crack does not split open. The rock pushes back on the skin with a force that is large enough to be painful, and which may even cut the skin if there are any sharp crystals in the crack. I was glad Mike was leading now, since he had wrapped his hands in adhesive tape to protect his skin from abrasion.
Skin is amazing.
The folds in the skin on the back of a knuckle allow the skin to unfold to 1.8 times its original length before it begins to stretch. It can then stretch elastically to more than double its original length before it breaks. When the hand is first jammed into a crack, like the one Mike was climbing, the skin will stretch and move, which can cause the hand to slip from the crack. Tape on a hand does not stretch. It anchors the skin in place, making the climbing of jam cracks more secure.