If you hold your lower belly (navel to pubic bone) flat as you diagonal crunch, you will be teaching trans abd and obliques to work together.

You'd still be training your muscles to fold you in half (not something we do much in the real world) but at least they'd be working together.

Planks and other ab-intensive exercises that are done while holding your spine in neutral win the gold medal in my view. After all, we want to train the muscles to stabilize us, not fold us in half.

Ok, everyone, plank time!