The defenders have to hit the shuttle farther, the net man has better chances of cutting the smash return off, and the defenders are more vulnerable to a varied attack where the offense mixes in drops and half-smashes. As the defender, you are hoping for the wide cross-court since it gives an opportunity for an outright winner. Jut your shoulder forward, get the elbow in front of you, and cock your wrist. The point here is to give the racket head room to swing so that you can meet the shuttle in front of you. The impact is both a snap or a flick at the shuttle with your wrist, as well as a push outward with your whole arm.
Many players choose either a backhand or forehand stance when they wait for the smash.
If you do choose a side, the backhand defense is much stronger than the forehand, which is like an open stance of a baseball hitter. If the pitch is thrown at the batter, or slightly behind him, there is nothing much the batter can do except try to get out of the way. So it is with badminton player smashing at a defender waiting on his forehand. The attacker can smash from the defender's outstretched elbow to anywhere on his body to anywhere near the backhand. At least with the backhand defense, the defender can protect his body.