The thumb grip is like the backhand version of a panhandle grip. It’s mainly used for backhands where the shuttle is in front of you. That means it’s useful for many backhands in the midcourt or at the net, but not in the rearcourt.
Let’s start with a forehand grip. From here, the racket should be turned about 90 degrees, making the strings face the floor. Place the thumb onto the racket handle, on the widest bevel.
If you now bring your hand up and bend back your wrist, the strings will face forwards. This is a typical position for using a thumb grip.
Since this grip is normally used for backhands, the pad of the thumb is placed directly onto the racket handle. This allows the thumb to support the racket from behind, providing stability.
However, the thumb should not be pressed flat and tight against the handle. There should be a gap between the handle and the lower part of the thumb.
The thumb should also be placed at a slight diagonal angle, rather than pointing straight up the handle.
As always the grip should be relaxed and cradled lightly in the fingers; it should not be held tightly in the palm.
Try bringing the index finger lower
The finger positions may be adjusted slightly too. Try bringing the index finger a bit lower, so that it’s closer to the other fingers. This means the index finger is placed lower on the handle than the thumb.
I find this more comfortable, and I feel it gives me better leverage when I tighten the grip. When the index finger is high, I feel it blocks the thumb and wrist movement, and creates tension in the hands.