Your abs are dead. Bring them back to life. Balance your full body weight atop the abdominal wall on a surface no wider or longer than the six pack. The height may be adjusted so as to avoid floor contact (video here: https://www.instagram.com/p/By7o8ZiDAq6/).
Recently I was playing around with optical illusions only to find that balancing stomach atop a yoga brick is harder than most core exercises, since there is NO way to cheat it. There is no momentum component. There is no hip flexor cheat. There is no fakery. You must create maximal intra-abdominal tension or it just won’t happen.
You might have watched or experienced taking a real punch or strike to the stomach. You can’t fake it. You have to contract the abs. This is like that except a longer-lived effort. And unlike the typical plank which requires ZERO abdominal activation, you are required to call on more than a phony bridging pretense.