I have an instructor who cues:" flex the obliques" when lifting the legs legs up and over and then flex the obliques to let the side make contact with the mat - I really cant understand this terminology - I understand the many ways to stretch the obliques but flex? Would like clarification and your opinion on this cue

My guess would be the instructor is telling you to contract or shorten the oblique. Initiate the movement by contracting your oblique muscles.