When climbing, friction is your best friend. It can enable even the most seemingly impossible moves when used correctly. To take full advantage of its spiderman-like properties, it is important to understand how it works.
First off, there are two different kinds of friction: static and kinetic. Static friction prevents objects from moving so long as the force exerted is less than the static friction. Once the force exceeds the amount of friction exerted, the object, or climber, begins to move. Kinetic friction occurs when friction is able to slow down an already moving object.
A climber creates friction by combing two essential ingredients, namely, themselves and the rock. When there is static friction, you can climb. As soon as the force generated by the climber surpasses the static friction - you fall.
In order to fully take advantage of friction while climbing, knowing how forces work in conjunction with finger positioning.
When applying a force with your fingers together, the forces generated from each finger are parallel to each other and perpendicular to the hand hold, which gives the climber maximum frictional force against which to pull.
With fingers separated, the forces generated in each finger are not parallel; rather they are at slight angles. Because of these angles, some part of the applied force will try to move the fingers together.
Of course, there are times when separating the fingers is necessary. Perhaps you find a hidden little thumb catch that turns that crappy crimp into a slightly-less-crappy pinch. Or maybe you can only fit two fingers in that bomber pocket along the chossy rail. Or maybe there’s that one spot on the almost-perfect jug where the rock is just too damn sharp!
While we lose some force by separating the fingers, we presumably gain more force (or at least, more useful force) from whatever advantage the new finger or hand orientation provides (turning a crimp into a pinch, for instance).
Typically, the more of your hand you can get on the rock the better. So, chalk up, and let's hit the crag.