The bevel grip works well for overhead backhands that are level with your body.
When the shuttle has travelled behind you, however, you need to shift towards a panhandle grip.
This change in angle is necessary in order to hit the shuttle straight. If you don’t adjust your grip, then the shuttle will go out at the side.
Placing the thumb on the side of the racket handle can help you gain some leverage for backhand power. This thumb placement is different from a normal panhandle grip, because now we’re using the grip for backhands.
Think of this as adjusting the angle of the bevel grip.