Reach side to side and crunch your obliques with every tap.
Crunch sideways from your obliques, do not just slide your arm along the floor.
Heel touches are a side to side crunch: your shoulders hover off the floor while you reach for one heel, then the other, and every tap comes from the obliques contracting on that side. One side works at a time, which keeps tension bouncing back and forth across your waist. Small movement, direct oblique work, no equipment needed.
Primarily the obliques, the muscles along the sides of your waist that bend your torso sideways. The rest of your core works to keep your shoulders hovering off the floor between taps.
You are pulling your head forward instead of holding your shoulders up with your core. Keep a fist of space under your chin, gaze at the ceiling and let the lift come from your trunk, not your neck.
Walk your feet further away so the reach is longer, slow every tap down, and keep your shoulder blades off the floor for the entire set. Constant tension does more than extra speed ever will.
