Tough question. They are electronic switches that bridge the power to the trigger contacts. They are many different kinds for many applications. They increase battery efficiency, prevent arching, damage to trigger contacts and motor ware (especially in long burst). They increase ROF and motor response. In addition to MOSFETs, they are commonly used with other switches to correct more complicated MB problems, like gear spin or dry firing. MOSFETs are generally inexpensive and are more desirable when using higher voltage setups.
this is the original article. I read this last year and these guys have been perfecting them for a while now.
http://www.airsoftmechanics.com/home...iew/23/26/1/0/
http://forums.airsoftmechanics.com/i...hp?topic=485.0
the future
http://forums.airsoftmechanics.com/i...hp?topic=648.0
I don't think I did justice to MOSFET wiring but hopefully I was able to convey the idea. If not, their forums have all the info you want.
mateba