some are transducers that are like pickups or mics, that attach to a drum and send out varying signals ( like D-Drum Trigger Packs )
some are actual pads that you hit with sticks( though not technically triggers, many refer to these, like Roland V-drum pads)
these are to record a performance, and not the SOUND of the drums so that later on they can fire off other drum samples to be determined at a later date, or sometimes to use creatively, like a snare drum trigger that goes into the sidechain of a ducking noise gate that the guitar is running thru...makes the guitar only heard when the snare is hit...
lmao uh yea...drummers seem to be finnicky, Im just basing this on ones I have known not all of them. The one Im working with now has this weird thing, half an hour into the jam he up and takes off to read a book...then comes back half an hour later...