Behind-the-Neck Lat Pulldown

Trains the Lats.

How to do it

Set a wide bar on a lat pulldown machine and adjust the thigh pads.
Take a wide overhand grip and sit down with your arms straight overhead.
Sit upright rather than leaning back — the bar has to clear the back of your head.
Tip your chin down slightly and pull the bar down behind your head toward the base of your neck.
Go only as far as the shoulders allow with the torso still upright, which for most people is not to the neck.
Let the bar rise under control until the arms are straight again.

Pulling the bar to the base of the neck regardless of what the shoulders allow. This variation asks for full external rotation and abduction at the same time, and how much of that is available varies a great deal between people. The bottom of the range is set by the shoulder, not by the neck.
Craning the head forward to make room. If the head has to move out of the way, the bar is travelling further than the shoulders can follow.
Choosing it over a front pulldown for the lats specifically. The latissimus dorsi shortens the same way whichever side of the head the bar passes; what changes is the shoulder position it is done in.
Using the same weight as a front pulldown. The position is less strong, so the load comes down with it.