How you start you sentence when you're about to recall an event or occurence that was funny, embarassing or awkward. Usually the moment you are recalling is something that has embarassed many.
the awkward moment when... you fart during a business conference and try to cover it up with a cough, but realise immediately and without need to confer, that everyone around you knows you just farted.
The awkward moment when... you make a black joke seconds before realising you are standing next to a black guy.
The awkward moment when... you say bye to someone then you both continue walking in the same direction.