Locked-in arms, constant tension, no cheating.
Keep your armpits pressed into the pad and never fully lock out at the bottom.
The preacher pad locks your upper arms in place so your biceps can't get help from swinging shoulders, and feeding it from a low cable instead of a bar means the tension never drops, not at the top squeeze and not near the bottom stretch. That pairing of strict positioning and constant load is the whole appeal. It earns its place when you want the most honest biceps curl in the gym, every rep from a dead-strict position with nowhere to hide.
The pad does the same job in both, killing momentum, but a barbell goes nearly weightless at the top of the curl while the cable keeps pulling the whole way. If you've only ever done free-weight preachers, the constant tension version is a rude awakening.
Fully locked out on a preacher pad puts the biceps tendon in its most vulnerable stretched position under load. Stop just short of straight, you keep tension on the muscle and stress off the tendon.
Yes, that's your shoulders escaping to help. Press your armpits into the pad, keep your chest against it, and if the contact keeps breaking, the stack is set heavier than your biceps can honestly handle.
