I attempted to explain how they work in another thread:

They have a coil spring inside. The spring has no effect when you pull the lever back, but it unwinds slightly when you push it forward. When a coil spring unwinds, it gets longer. The longer spring increases the amount of friction by just enough to offset the pull of the derailer spring. When set up correctly, the shift levers move easily in both directions and they always stay where you put them.

It hasn't happened to me, but the spring can break. If it does, you will have standard friction shifters and you'll need a screwdriver to increase the friction.