The "hook grip"
Hook Grip in weightlifting defines the method in which an athlete grips the barbell, which is first gripped with the thumb, then the remaining fingers grip both the thumb and the Barbell. It can be used in multiple weightlifting events (i.e. in “SNATCH” and “CLEAN AND JERK”).
The Hook grip is considered to be a more secure grip compared to a Closed grip or Natural grip where the thumb remains outside the other fingers. Also, history of this sport reveals that athletes who use the Hook grip can lift more weight compared to those who use a Closed grip. The Hook grip may not be suitable for beginners as the thumb or thumbnail may suffer pain, although this can be overcome by regular training.
The following is an excerpt from Greg Everett’s Olympic Weightlifting: A Complete Guide for Athletes & Coaches
The hook grip is a pronated (palms facing the lifter) grip in which the thumb is trapped between the bar and usually the first and second fingers, depending on hand size. For the pull of both the snatch and the clean, this method of gripping is a necessity to maintain control of the barbell during the violent second pull and the powerful turnover of the snatch.
It’s important to understand that the thumb is itself wrapped around the bar inside the fingers and not simply pinned perpendicularly to the bar. This arrangement takes advantage of the greater strength of the thumb relative to the fingers–with the thumb wrapped over the fingers as it would be in a conventional grip, it will typically reach only the index finger and have a weak purchase on it.
By wrapping the thumb around the bar directly, we create a powerful hook on the bar, which can be reinforced by the grip of both the index and middle fingers, which serve more to support the hook of the thumb than to grip the bar directly. With two fingers over the thumb rather than only a weak section of the thumb over one finger, we also create far more friction to secure the grip. In short, the Hook grip optimizes the anatomy of the hands for this application.
Particularly in the snatch, the grip will be reliant almost exclusively on the thumb, first, and second fingers. The wide hand placement of the lift results in an acute angle where the hands attach to the bar, forcing the third and fourth fingers to have little purchase.
In the case of these two lifts, the integrity of the grip of the first and second fingers in combination with the thumb is critical. In order to ensure this integrity, the athlete needs to use the fingers to actively pull the thumb around the bar rather than simply pressing it against the bar. The hook of the thumb under the bar with the fingers reinforcing it is what provides the necessary grip power. The force of the fingers on the thumb should be the focus of the athlete’s gripping effort.
Typically the Hook grip will be uncomfortable if not considerably painful initially. Consistent use will condition the offending structures appropriately over time and the grip will ultimately offer no trouble. It will, in fact, become more comfortable than a conventional overhand grip. Covering the thumbs with flexible athletic tape can reduce the discomfort and, for some, improve the feeling of grip security by increasing friction. Lifters can submerge the hands in ice water for 5-10 minutes after training to help reduce pain and speed the adaptation.