Systema Revolution gearbox
 

Is anyone familiar with this gearbox.

How does the trigger lock when "safe" is selected?

I've installed it a couple of times, but I can't for the life of me remember the operation.

I remember it was electronically controlled. As far as I can recall, putting the lever in to safety disables the gun by breaking the circuit. As long as the circuit's open, it doesn't matter how many times you pull the trigger, no electrical current will go through.