Amber Rose and Blac Chyna -- This Is How We Do It ... Amateur Gynecology

Amber Rose and Blac Chyna let their fingers do a lot more than walking outside a Hollywood nightclub -- in fact, we can't really say what they were doing ... because we're too classy.

The ex-stripper BFFs left the Ludacris album release party at Project Club LA and hopped into the same ride ... which was apparently so packed they had to lap up -- with Amber on top.

We'll just say they kinda held an impromptu sex ed class for fans and photogs -- and everyone paid very close attention.

Moms MILFS do the darndest things.