Load it heavy, build arms that fill sleeves.
Keep your elbows locked at your sides, no swinging from the hips.
The barbell curl is loaded elbow flexion at its most stripped back: underhand grip, elbows nailed to your ribs, bar travelling from a dead hang at full stretch to a hard squeeze at the chest. Both arms share one bar, so there is nowhere for a lazy bicep to hide. The lifts that fill your logbook with small, steady weight jumps are the ones that build arms, and this is the classic example.
Heavy enough to challenge the biceps, light enough that your hips and lower back stay out of it. The moment you are bouncing at the knees to start the rep, the weight is running the set instead of you.
Yes, lower to a full stretch each rep. Stopping halfway keeps constant tension but shrinks the range the biceps grow through. Full stretch, controlled lift, hard squeeze, that is the complete rep.
As a deliberate tool at the very end of a set to push past failure, maybe. As your default technique, no. If every rep needs a hip swing, the bar is too heavy to build anything but bad habits.
