Unlike some other exercises you should not pause when your arms are bent, this is basically resting at the bottom of the movement.
Keep the exercise flowing and control the weight.
Keep your elbows in and upper arms fixed during the set.
Use a very light weight as first!
Instructions
The lying barbell reverse extension is a great exercise to truly isolate the triceps! Set up by sitting on the end of a flat bench with the barbell or EZ bar resting on your thighs.
Grip the bar with an overhand grip (palms facing down) with your hands about 8-12 inches apart.
Lay back on the bench and extend your arms out behind your head so that they're close to parallel to the floor. This is the starting position for the exercise.
Keeping your elbows and upper arms fixed in place, slowly lower the barbell behind your head.
Don't pause, and then extend your arms to bring the bar back to the starting position.