The difference between a sit-up that works your abs and one that cranks your neck is keeping your neck neutral. Lie on your back with your knees bent and feet anchored. Your hands can be behind your head, but don't pull your head forward — just guide your fingertips there. The movement comes from your core, not your arms or your neck. Roll your shoulders up toward your hips one vertebra at a time, pause at the top, then lower back down slowly. If your neck is sore after, you're pulling too hard.