Just lifting the butt will not really make a plank much harder, because there is more of your weight on hands/elbows and feet. It will make it dynamic instead of static, which is harder to stabilize though, comparable to doing it on a ball/ring etc.

The real deal is no directly viewable from the outside though:

You have to turn the yielding isometric (standard plank, resist the trunk falling down to the floor) into an overcoming one (trying to squeeze the floor between arms and legs together). That's often called an RKC plank (after that Pavel kettlebell guru ex-company). You can read more about yielding vs. overcoming isometrics here, for example. Focus is on BB, bu it's about the categories: https://www.t-nation.com/training/isometrics-for-mass