Imagine the following. You've got a battery with a +ve (red) and -ve (black) connections on it. You run a black wired from the battery (-ve contact) straight to the -ve terminal of the electric motor.
If you ran a wire straight from the +ve (red) terminal of the battery and touched the wire to the +ve terminal of the motor...the motor would spin, because you've completed the circuit.
All the switch assembly does is put an On/Off switch in the middle of that red wire. Basically there are two metal contacts...and the trigger pushes a metal block inbetween them to complete the circuit. Imagine a mechanical throw switch (like the old movies where the warden throws a big switch to fry a guy on an electric chair....same sort of thing, a mechanical contact switch).
The plastic housing just holds the 2 metal contacts, metal block, return spring and connecting wires in place.
When the circuit is completed...the motor spins...turns the gears...pulls back the piston...and the shot is fired.
Some switch assemblies have metal contacts on their side (usually the left side)...and the circuit is completed by a metal tab on the selector plate. This is a bit redundant since completing the circuit there does not cause the motor to spin...and the safety is a mechanical block that prevents the trigger from moving. But I suppose it is a redundant safety...so whatever that's worth.
How the mechbox is put into semi-auto vs. full-auto is an entirely different thing....but essentially a little arm flicks the metal block out of contact after each shot.
Hope that helps,
Last edited by m102404; September 29th, 2009 at 11:47..