You can model this phenom by thinking of a jar of peanut butter. The lid is your cog, viewed from the bikes driveside. To "pedal forwards", just tighten the lid. To "resist", turn the jar clockwise while holding the lid stationary. Yep, the lid comes off, because peanut butter jars do not come with a reverse threaded lockring.


Doof is right though, the guy was probably tryiong to sell you a lockring wrench, which you should get if you don't have one. Vigorous forward pedalling by you will tend to tighten your cog even more than it already is, effectively loosening the lockring. Then, next time you resis the cog will spin back to where it was, slamming into the lr and potentially stripping the threads. Lots of people ruin hubs this way.

A lockring wrench fits in those little notches i pointed out to ya. Remember, the lr tightens backwards!