First off, if you want a detailed explanation of combination locks, pop over to Wikipedia
Imagine a Master combination lock as simply a a number of discs (or rotating dials) with notches in their inner circumference. These notches correspond to a particular number on the outside of the disc (the part we use to unlock it). When the correct number sequence is inputed, the notches line up, and the bolt is released.
The most important things to know are (1) all 4 numbers must be correct to release the bolt (2) as we can't see the notches, we have to rely on touch to feel when a notch is engaged or not, (3) in order to feel when a notch is engaged, we need to provide upward pressure to the bolt, and (4) in order to feel each number out correctly, we must start from the bottom up as this will allow us to know when each subsequent notch (up the lock towards the bolt) are engaged.
Enough of this. Lets pop a lock.