You know how when you screw in a bottle you lose a bit of air because the pin is depressed before the bottle is screwed in all the way and sealed? An OOPS only depresses the pin when you switch it on so you can screw the bottle in without losing any air or damaging o-rings. It also vents the air in the macro line when you switch it off (but your reg may still have a few shots in it afterwards so make sure you discharge the reg safely). That's how it was explained to me anyway. Hope this helped (from one newb to another)