Check the badminton bible on how to grip the handle for a smash. Also if you want more power go with a thicker grip.
The grip may vary depending on where the shuttle is: in front of you, level with you, or behind you. Here are a couple of relevant pages from my grips guide: Smash grip adjustment Which grip for clears, smashes, and drops?
Yup. I read most of the ones on Badminton Bible. They are awesome. Thanks! EDIT: It tells me to have a relaxed grip, but I used to grab it very tight. It'll take time for me to get used to the new one. If I have a relaxed grip, won't my racket fly off while swinging?
Why, thank you. Glad you found them helpful. You need to tighten your grip during the swing. As well as preventing your racket from flying across the hall, this grip tightening will make your shot more powerful.
When smashing you should have strong grip when smashing. If you have a loose grip there is a chance that your racket might fly off your hand.
Experiment. It's possible for the grip to be too loose, and it's possible for the grip to be too tight. The tightness of the grip varies at different stages in the shot, and it varies depending on what shot you're playing. The grip does need to be tightened somewhat when the racket is moving fast, or you'll lose control: the racket will slip and turn in your fingers. It may even fly out your hand. However, try to keep a relaxed grip before your shot, and during the early part of a swing. You want to save at least a little tightening for the end.