From my basic understanding of locks (I only just got a lockpicking book for Christmas and haven't read through it all yet) is that we depend on one thing to pick this kind of lock: machine error in construction. Because of this the pins won't be in a perfect line. We use a tension tool to apply turning force (in the same way the key would be turning it) on the lock, and because the pins aren't in a perfect line, the lock will really only be stopped by one of the pins. You then use a different tool (usually a hook in this case I think) at the same time to poke at the pins to figure out which one is preventing the lock from turning. At this point, you fiddle with the locking pin so that it lines up with where it should be as if the key was inserted, and that will cause the two parts of the pin to be in the desired 'open' state. The tension tool comes into effect here, because you'll slightly rotate the inside of the lock with it so that the pin you just 'unlocked' no longer falls back into place. It has now been dealt with, and as long as pressure is being applied to keep the lock turned it will stay so. Then you move on to the next pin that's blocking the lock and so on, until all the pins are up and you've unlocked it!

Disclaimer: as stated before, I am a newb at lockpicking and this is only my interpretation of a small bit of reading. Have fun trying it out though if you do!