Lock your arm to the pad and isolate every fiber.
Don't fully lock out at the bottom, keep a slight bend to protect the elbow.
Anchoring the back of one arm on the preacher pad removes every trick in the cheat book, no swing, no elbow drift, no shoulder lift. That leaves a single biceps to curl the dumbbell through a strict arc, with the pad putting real tension on the stretched bottom position most curls skip. It earns its place as the honesty test for your biceps, and the go-to when one arm has quietly fallen behind the other.
Stop just short of a hard lockout. The pad puts the stretched position under heavy tension, so keep a slight elbow bend at the bottom and lower into it slowly.
Set the seat so your armpit sits snug against the top of the pad and the back of your upper arm stays in full contact throughout. If your elbow lifts off mid-rep, the seat is too low.
Because the pad deleted your momentum. Standing curls let your hips and shoulders chip in, the preacher makes the biceps do every millimetre, so the dumbbell has to be lighter.
