Indirectly you actually are stimulating the abs. Even though it is not a direct hit like sit-ups. Kinda like before a contest its recomended that you flex for at least 30-45min a day. Some pros, like arnold once said he flexed everyday after workouts especially to bring out those extra fibers that make striations appear. I myself flex every morning trying to flex ever fiber I can feel.
First I start with my feet, tense my calves, quads, hams, glutes, lower back(lumbar region), exhaling, abs serratus, forearms, biceps and triceps, in a downward motion, then as I lift my arms delts, inner,outer, back, and lastly chest into a most muscular pose. And hold it as long as I can.
Sometimes I inhale and tense all and sometimes I exhale and tense all. Ultimately it' s having control of as many muscle that you can contract no matter if you inhale or exhale. IN that case you can tell from the ones that dont flex often because when asked to perform a particular pose they shake and cant keep steady (*No control*)
It s so very obvious that most pro's these day don't exhale
while various poses,Like most muscular because it opens up the chest girth therefore appearing wide and long. Leaving it hard for them to suck there stomachs in Hence the belly ban.
Just look at shots of the oldies. When doing a most muscular for example they exhale making the stomach vacume and then expanding the chest and lats.
It just looks different. Look at ronnie for example that guy so huge that posing is almost effort less for him. But ask him to suck that gut in and then make a most musclular pose?
I'm not saying he can't but ive never seen it.