In reference to this video (at about minute 6): Is the opposite/reverse plank good for abs, and can it help to gain a 6-pack?
The opposite plank places load on the posterior chain. The body ‘wants’ to sag downward under its own weight, thereby lengthening the contractile (muscle) and elastic (elastin in the connective tissues) structures of the back, and correspondingly shortening those structures in the front of the body. That tendency is counteracted by the muscles of the posterior and, of course, plastic and elastic forces (most notably from collagen and elastin, respectively) of their connective tissues.
The primary muscle involved are the erector spinae, gluteals (maximus, medius, and minimus), and hamstrings. We can (and do) indeed contract the rectus abdominis, internal and external obliques, to control those muscles, but this action is antagonistic and largely independent of the loads being borne by the prime movers. That is, development of the abdominal muscles is largely independent of the loads being applied in this exercise. In this regard, the opposite plank should not be considered to be an abdominal exercise.
I hope that is helpful.