If you understand the principles behind grips, it will help you make sense of the details later.
A relaxed grip
Grips should be relaxed, not tight. A relaxed grip helps the muscles of the forearm work well. Most club players hold the racket too tightly, causing their arm muscles to lock up during the stroke, so they lose power.
During the stroke the grip will tighten, particularly in a power stroke such as a smash. The grip will go from relaxed at the start to very tight at the moment of impact. The change from relaxed to tight is essential for transferring power into the shuttle. You can hear the difference between a relaxed and tight grip as the racket moves through the air. The relaxed grip also makes it quicker to change grips.
You can test how relaxed your grip is: ask a partner to take the racket out of your hand. The racket should come out your hand with only a little resistance. Just be careful not to rig the test!
Small gaps between your fingers will make the grip more sensitive. Fingers clumped together will give you less control of your racket face angle. The gaps may reduce or close as the grip tightens.
A good grip will let you control the angle of the racket face as you hit the shuttle. So for a straight shot, the racket face will be pointing straight forward at the moment of impact — but note that the racket face direction will change during the stroke, often because of arm rotation.
There are exceptions to this: when using slice, you deliberately hit the shuttle at an angle. But it’s still important to control the racket head angle and control the amount of slice. A bad grip can cause an accidental slice, where you lose power and the shuttle goes in the wrong direction.
Using a long grip
As a default, you should hold the racket in a long grip, with the bottom of your hand near the base of the racket handle. This effectively creates a longer lever, and with more leverage you get more power.
Don’t go too far though: make sure all fingers are on the handle!
The last two fingers
When you need to hold the racket firmly, rely on the last two fingers (ring finger and little finger). These two alone stop the racket coming out of your hand in a powerful stroke, or wobbling about. As a rule, these two fingers should be holding more tightly than the others, they provide stability. The other fingers are for subtler manipulations.
Forehand and backhand differences
For forehand shots the thumb tip will not be completely on the racket handle: the contact point will be on the inside of the thumb. This allows the thumb to curl around the handle in a powerful stroke. There will usually be a larger gap between the forefinger and middle finger than between other fingers.
In a backhand the thumb is placed directly onto the racket handle, to give support to your backhand shots. You also have a larger space between the handle and the heel of the thumb. The gap between the forefinger and middle finger is reduced, or closed. When you hit a backhand shot, even a powerful one, the thumb stays straight.