The mistake that sneaks in here is using your shoulder to start the curl, rocking your body back and swinging the handle up. Stand facing the cable with one hand on the single handle, elbow pinned to your side, and take a small step back so there's tension on the cable even with your arm straight. Curl the handle up toward your shoulder using only your elbow as the hinge, keeping your upper arm still against your ribs. Squeeze at the top, then lower slowly — the cable keeps constant tension through the whole range, unlike a dumbbell, so the stretch at the bottom is where you'll feel it working hardest. Finish one side before switching hands.