Rambling about hairtrigger and overspin with DSG (airsoft) - YouTube
(part2) Rambling about hairtrigger and overspin with DSG (airsoft) - YouTube
you'll have to pardon his rambling, it's not usually his style, but all the information is in there.
(also google suzaku's hair trigger guide, I think he had the link in the video description)
I didn't do it quite the same way or to the extreme that he did (didn't have the same materials on hand), but also because I didn't want a trigger pull quite that short (mine ended up being about 1.2-1.5 mm, still way shorter than I wanted...)
it's all done with the same trigger trolley setup. no microswitches or otherwise. there's definitely no feeling of a "break" like a GBB or "activation" like a switch, but the whole thing going off at 30+ rps tends to do that for you.