Plank

You let the hips sag toward the floor or pike them up to make the hold easier.

You hold your breath instead of breathing steadily behind the brace.

You chase long hold times with sloppy form instead of shorter holds done perfectly.

Squeeze the glutes and tuck the pelvis slightly so the lower back stays out of the exercise.

Actively push the floor away through the forearms rather than hanging on the shoulders.

Treat the plank as a full-body squeeze; the harder you brace, the more you get from it.