Use a full range of motion on this exercise but do not curl the weight up too far. Once your forearms are at an angle of more than 90 degrees to the floor, youre essentially resting.
As always, use slow and controlled rep timing.
Instructions
Setup for the EZ bar reverse grip preacher curl by adjusting the seat height of the preacher bench so that the back of your upper arms rest flat on the pads.
Load up the bar with the desired weight and sit on the bench.
Grasp the bar with an overhand grip (palms facing down).
Keeping your arms fixed, and only bending at the elbows, curl the weight until your forearms are at 90 degrees to the floor.
Do not pause, and slowly lower the weight back to the starting position.