I really like Christine’s answer.
We do a lot of core work in yoga. The way we would refer to the use of intraabdominal pressure, as in an isometric, is mulha bandha. Plank is great for getting this, but then keeping it while adding the dynamic (the flow) is better than just holding it for longer. One type of progression I sometimes use is the half sway…. there is a position called vasisthasana, or side plank…. I will do a half plank on the elbows and keeping the feet together as a unit roll slightly and then work to keep the hips straight, and then add lifting one arm, and then move to straight arms, and then to rolling to full side plank and lift the upper leg as well as the upper arm. I also will sometimes cue to imagine the feet are tied together and say to alternate straightening each foot and pointing the toe without moving the leg, keeping the ankles together.