Locked-in arc, pure bicep tension.
Keep your upper arms flat on the pad and don't let the weight yank your arms straight.
The machine curl locks your upper arms on the pad so the biceps curl through a fixed arc with zero help from swinging hips or drifting elbows. The cam keeps tension on the muscle at the top squeeze and through the negative, the two places free-weight curls tend to leak. It's the place to take your biceps to honest failure safely, especially at the end of an arm session when strict barbell form has already gone.
For pure bicep tension and training close to failure, they're excellent, the pad removes every cheat. Free weights still earn their place for stabiliser work, so treat the machine as a partner, not a replacement.
So your armpits sit snug at the top of the pad and your elbows line up with the machine's pivot. If your elbows sit above or below the pivot, the arc fights your arms instead of matching them.
Go to a full stretch but stay in control as you get there. Letting the stack yank your arms straight unloads the biceps at speed, and that snap at the bottom is where sloppy reps go wrong.
