People do not understand abs exercise. I see this when people say they are into gym training, and do abs to tone up. First of, of course, if your stomach is toned, u do not flag in any yoga pose... they are related, your core strength and overall stamina, obviously.
When I ask people to show me what abs they do, they show me a move which is universal -- the legs folded at the knees and hands under the head and lifting the torso up and down. But they take the brunt of the pose at the back, depending on their weaknesses, they will shift the weight to the strongest part of the back, so neither does the back tone up, nor definitely, the stomach.
For proper abs, you need to lift up from the belly, with the pressure shifting to the thighs. The hands under the head should have the elbows pointed out, ideally both for the up and down movement. But if that is difficult initially, at least for lowering movement of the back. If pressure is being felt at the neck or lower back, your stomach is def not toning! Plus, the stomach should be deeply drawn in while lifting up, with an exhalation. This synchronisation of breath with muscular effort is the most important factor in real abs. You do not need to hundred, as some stupid gym sessions suggest you to: but 20 without any rest break, with the above suggestions, will explain the difference between men and boys:)