The block shot is generally used in defense against an opponents smash, but if a player is up at the net it may also be used against a drive shot and can become an attacking shot.
If a player plays a block shot at the net with the racket face closed (pointing at the ground) then this will send the shuttle down quickly making it difficult for the opponent to return.This shot can only be played from the mid court to the net as it uses the power on the opponents shot to take it over the net. If a player tried to play it from the back of the court it would not reach the net unless they swung the racket and then they would be playing a different shot.
Players should already be in the ready position waiting for the opponents return. From this position watch the shuttle off the opponents racket and quickly move in line with the shot. Make sure the racket will intercept the shuttle, and depending on the court position and the height of the return adjust the racket face accordingly. For example if at the net a player may be able to point the face down, however if they are mid court and trying to block a smash they may have to open the face. This will enable the shuttle to clear the net.
The racket should not be swung at all, it is there just to block the shuttle taking all the pace out of the opponents smash or drive. With this shot because the shuttle is traveling so quickly at a player, it will leave little time to assume the correct body position. Players can be blocking the shuttle from peculiar positions but the best way would be to stand square facing the net with the racket out in front of the body.