Blac Chyna Judges Twerking Contest in Atlantic City

Blac Chyna surrounded herself with all kinds of asses -- it's really the only way to properly judge a twerking contest.

BC was at the newly renovated Pool After Dark at Harrah's in Atlantic City over the weekend to rule on important stuff ... like asses shaking. One competitor busted a huge no-no -- and security even had to step in to remind her to keep it clean.

But you gotta see Chyna's face when it went down.