When your opponent hits the shuttle about two-thirds forward you should smash. In doubles you can smash from the back. When doing the smash, you need to turn sideways using your torso. Reach up, twist your torso and swing across your body. This puts all your power into the racket. It is like thrown a ball downwards. Make sure the contact the shuttle in front of your body. Also, when you are smashing, it is important not to follow the shuttle all the way through, let it disappear forward. If you drop your head, the shot will be too low.