Tiny reaches, massive oblique burn.
Bend through your side, not your shoulders. Keep your shoulders off the floor the whole set.
With your shoulders held off the floor, reaching side to side for each heel bends your torso laterally, which is precisely the job of the sides of your core. The constant crunch position keeps the front of the core switched on throughout, while the alternating taps add the side-bending work most ab routines skip entirely. It earns its place as a low-skill, no-equipment oblique burner that anyone can drop at the end of a session without needing a bar, bench or mat setup.
Down the sides of your waist, with a steady burn across the front from holding the crunch. If you mostly feel your neck, your shoulders have dropped to the floor and you are yanking your head instead of side-bending.
They are safer to learn. Heel taps bend the trunk sideways with your back supported, while Russian twists add rotation that beginners often turn into arm swinging. Start with taps, add twists once you can control them.
Slide your feet further away so the reach gets longer, slow every tap down, or hold the crunch higher off the floor. Adding speed just invites momentum, so make the range and tension bigger instead.
