Fixed arc, constant tension on the biceps.
Keep your arms planted on the pad and control the negative, don't let the stack drop.
The machine preacher curl locks your upper arms on the pad and moves the load through a fixed arc, keeping your biceps under tension from the stretched start to the squeeze at the top. There is no balancing and nowhere to cheat, and the pin-loaded stack makes drop sets take seconds. It is the easiest way in the gym to take biceps to true failure safely.
For pure biceps stimulus, yes, and the fixed arc plus the stack makes training to failure and running drop sets far more practical. Free-weight versions simply add a little more stabilisation work.
So your armpits sit snug against the top of the pad and your elbows line up with the machine's pivot. Too high or too low and the arc fights your elbow instead of matching it.
Letting the stack yank their arms back down between reps. The lowering half of the rep matters as much as the lift, so control the negative for a slow two or three count.
