Hips must be forward of the pad. This is a pure hinge maneuver, so make sure your ability to flex and extended from the hips isn't impeded by the pad.
Seek maximum range of motion. Go all the way down and all the way up to parallel.
Match lifting tempo to resistance curve. As you move through extension, start the concentric rep somewhat slowly, and then accelerate as you move towards full extension to compensate for worsening leverages.
Keep your entire spine neutral, including your neck (don't look up as you reach full extension).