Lets talk about what happens when a lock is unlocked. First the key is inserted in the lock. This pushes the pins so they are now positioned above the cylinder. The line that the pins need to be pushed up to in order to successfully unlock the lock is called the sheer line. Now that the cylinder is pin-free the cylinder is free to move, and the lock may be unlocked. Here is a picture to give you a better idea of what the parts are.
Now that we know how a lock is supposed to work we can learn we can exploit it. In order to unlock the lock we need to get those annoying pins out of the way. However we have no way of knowing how many pins there are, or how far they need to be pushed to exceed the sheer line. Luckily for us there is a is a really simple way to get around this. When tension is applied to the cylinder, it causes it to move slightly, creating a kinda shelf. If the pins are pushed up while tension is being applied the pins can rest on this shelf, even what the object pushing them up is removed. If all the pins are pushed up on the shelf then there is nothing stopping the cylinder from moving and the lock can be opened without a key.
So thats pretty much the basics of how lockpicking works. Next post I will describe how you can apply this knowledge and common tools that are used.
