Every time a gearbox cycles on semi the cutoff lever breaks the connection between the battery and the motor causing the motor to stop. An AB mosfet will reverse the polarity of a motor and quickly stop it.
Always always always run a AB mosfet with a anti reversal latch no matter what your setup. At least on higher spring or lower ratio setups the motor will not be able to hold the position and it will unwind slightly (not all mosfets will stop at the beginning of every cycle consistently) which generates heat on the mosfet. Do this a bunch of times and the mosfet will fail prematurely.
Low ratio setups/ high voltage setups still generate some degree of over spin.
There are three styles of mosfet: a basic, active breaking (some times full featured), and mosfets which replace the trigger contacts on your aeg.
Basic mosfets simply bypass the trigger switch and run directly to the motor. They do this using a transistor and having the gate wired on your trigger contacts.
Active braking mosfets do the same as basic mosfets but they brake when the cut off lever signals the end of cycle.
^just reiterating what Sanguinor said.
The final style completely replaces your V.2(there are only v2 models of this at the moment) trigger contacts with something similar to a micro switch. This counts each cycle your aeg makes via the cutoff lever and is able to precisely apply burst fire and more will allow for a better stop at the end of the cycle. These however are the most involved when installing them into your gearbox. Due to different tolerances you can have problems with the trigger not contacting the switch enough, same with the cutoff lever, the selector plate can also not contact the switch enough to let it know what mode it is in, and the switch may need to have its edges trimmed slightly to fit the gearbox shell.
All that aside you may be wondering about the prices of these mosfets.
Generally a standard mosfet will run you 20USD or less.
A basic AB mosfet will run you about 20USD.
A full featured mosfet will cost anywhere from 40USD to 100USD.
Mosfet trigger systems are about 100USD.
I currently run a mosfet trigger system in my m4 based system and it has been absolutely the best purchase I have ever made in airsoft. After overcoming the initial hurdles to get it to work I really would not ask for more. The main thing that I like about it over everything I stated earlier is the fact the trigger pull is minuscule compared to a standard setup. This allows me to have a run semi as quickly as a polarstar and systema.