It will feel like it’s ripping your thumbs straight off to begin with, but in the long run it will improve your lifting.
First of all, the hook grip is a technique used by weightlifters to secure a solid grip on the bar. It involves the same principle as the alternated grip powerlifters use for their deadlift (one hand pronated and the other supinated). Before explaining the hookgrip, let’s talk about the basic double overhand non-hookgrip technique first.
In the double overhand grip the bar security is mainly relying on the fingers -- with the exception of the thumb -- which all open in the same direction, creating a tendency for the bar to roll out of the hand. With the alternated grip you're making the bar roll out of one hand towards the body (pronated hand) with one hand and away from the body with the other (supinated hand), leaving the bar nowhere to go.
The same idea is happening with the hook grip. Because the thumb is secured around the bar and the index and middle finger are covering it the thumb is making the bar go in one direction and the fingers in the opposite, making the spin of the bar a less significant factor in grip loss.
How will the hook grip improve technique?
Because you’re not relying on your hands to squeeze the bar for a secure grip, the rest of the arm is by default going to be in a more relaxed state, making it easier to move them where you’d like them to go -- Elbows up and along the bar during the pull under the bar.