In it's simplest form a mech mod (mechanical mod) is a battery tube with a 510/ego connector and a switch. It works the same way a flashlight works. Hit the switch and full battery power is provided to the atomizer coil inside your ejuice tank. It will continue firing as long as the switch is on.
Regulated mods (vv/vw mods), are of similar construction but control the voltage and limit the amount of current provided to the tank atomizer. They also include safety features like the 10 sec cutoff.