It's full name is "Rock Bell on Shoulders and Jerk Behind the Neck."
The bar is stood one one end. The lifter bends to place the bar in a comfortable position as he squats down to allow the bar to slowly and carefully fall into position. This will put him in a low squat. From here, he stands up. When ready, he dips, and keeping his head looking down a little, jerks the weight overhead with a split. After bringing the feet in line, and getting the down signal, the weight is returned to the platform in the normal manner; in front of the lifter.
With practice, most lifters can probably lift more this way than in the regular Clean & Jerk. However, a tall lifter will have a difficult time getting low enough to tip the bar over.
The IAWA and USAWA have a lift called the Steinborn. It involves placing the bar across the shoulders the same way, and then doing a squat. The rules stipulate a helper may place his foot against the bar so it doesn't slide across the platform. The same method should probably be used in the Jerk Behind the Neck.