Grab the pull-up bar with the palms facing forward using a wide grip.
As you have both arms extended in front of you holding the bar, bring your torso forward and head so that there is an imaginary line from the pull-up bar to the back of your neck. This is your starting position.
Pull your torso up until the bar is near the back of your neck. To do this, draw the shoulders and upper arms down and back while slightly leaning your head forward. Exhale as you perform this portion of the movement. Tip: Concentrate on squeezing the back muscles once you reach the full contracted position. The upper torso should remain stationary as it moves through space and only the arms should move. The forearms should do no other work other than hold the bar.
After a second on the contracted position, start to inhale and slowly lower your torso back to the starting position when your arms are fully extended and the lats are fully stretched.
Repeat this motion for the prescribed amount of repetitions.
Caution: The behind the neck variation can be hard on the rotator cuff due to the hyperextension created by bringing the bar behind the neck. Therefore if you have rotator cuff issues, use the pull-up to front version instead.
If you are new at this exercise and do not have the strength to perform it, use a chin assist machine if available. These machines use weight to help you push your bodyweight.
Otherwise, a spotter holding your legs can help.
This is like a normal wide-grip pull-up but you finish with the bar behind your neck instead of in front.