The hook grip is a method of holding a bar by gripping the thumb between the bar and the remaining fingers. It can be used during any CrossFit lifting moverment involving a barbell. (Snatch, Clean & Jerk, and Deadlift.
In CrossFit the hook grip is more secure than other or natural grips and allows an athlete to maintain a grip on the bar during the phase of highest bar acceleration, the second pull. The hook grip prevents the bar from rolling in the hands, as it would have a tendency to roll towards the fingertips in a normal overhand grip.
The hook grip places a large amount of pressure on the thumb and you may want to consider taping your thumbs.