Explanation: The word "under" is taken literally in the four theme pairs, making them OVER/under BETS, DOYOU underSTAND, STAYS underCOVER, and LACE underWEAR.
Fun theme that worked really well! NW corner was tough for me, especially 1D - I kept pushing for coffee or that kind of shot. And my heart was set on Beer Tasters being a time honored gov’t job!