The hook grip is a weightlifting grip widely used by powerlifting athletes in order to maintain better stability. In a hook grip you have to wrap your thumb around the bar, then grab your thumb with your first two fingers and pull it further around the bar. The hook grip can be used in various lifts including the snatch, clean and jerk, deadlift etc. A lot of athletes earlier preferred the mixed grip in lifts like the deadlift where a mixed grip is said to be more secure but the mixed grip has disadvantages of creating imbalances in the body. The hook grip is said to be more secure than the grips where our thumbs remain outside the fingers.
As the grip provides more security and lets you feel comfortable over a period of time practicing the grip, your weights also tend to go up with it. As far as discomfort is concerned, the grip will hurt in the initial days of your starting to practice it, but as you get accustomed to the grip it gets better to lift.
For a lot of people there is some amount of stretching that is required before they start lifting heavy on the hook grip style. The best way to realize this is to simply use the hook grip every time you’re pulling a bar. Your thumbs will stretch out a bit and your hands will become trained to the position.