Help me understand breach?

How exactly does it work when I open one? The little hand things disappear

The hand things are chest like items, you must click on them while the breech is open. If breech closes before you click on it then you lose it.

Yeah but they do have some interesting droppage going on, so well worth it.

It seems you need to have a good ranged/AoE spell/attack as well a movement trick or two to keep the breach open longer. Even more fun mechanic than essences, which I loved... but still have not run into any so far.