When you reload it emits a burst of electricity. The way I understand it is the more bullets you have to reload the bigger electric burst. 1 bullet does practically nothing but the effect, 50 bullets and it's a nice burst. I haven't had the chance yet, but I feel like reloading the Death Machine would be quite a substantial burst, unless of course there's a limit to the range possible that I'm not aware of.
Also when you're downed a large burst is released. Sort of nice. Makes getting the achievement for killing 10 zombies at once with the perk alone a lot easier.
The main purpose in the game is to use it when you are stuck, because there are so many nasty small places and corners on MOTD. Whenever you are surrounded you just shoot a little and reload and keep going until you get yourself unstuck, it doesn't work 100% of the time but with the upgraded acidgat it is almost unstoppable.