3/4 Sit-up

Lie flat on your back with your knees bent and feet flat on the ground.

Place your hands behind your head with your elbows pointing outwards.

Engaging your abs, curl your upper body up and forward, stopping around three-quarters of the way to fully upright so your torso reaches roughly a 60 to 70 degree angle without sitting all the way up.

Pause briefly at the top while keeping constant tension in your abs, then slowly lower back down without letting your shoulder blades fully rest on the ground.

Repeat for the desired number of repetitions.

Recruiting larger muscle groups to "help" move the weight instead of isolating Abs — swinging momentum or shifting the torso takes tension off the muscle this exercise is meant to train.

Flaring the elbows too wide or letting the hips sag/pike, which shifts load off Abs and stresses the shoulder joint.

Rushing through reps using momentum rather than controlling both the lifting and lowering phase, which is especially important since there's no external load to force good tempo.

Skipping a warm-up set at lighter load/intensity — even "beginner" exercises benefit from a rehearsal set to groove the movement pattern before working sets.