The backhand smash is one of the most difficult shot to make in badminton. Most players struggle to play this shot. There is almost no defense on a well executed backhand smash.
Put the inner side of the thumb at areas. Thumb to be at the larger side. This is to generate a force but not a spinning force.
Racket face surface facing towards you. Palm centered where the index finger and the middle finger have space between each other. Do not have all four fingers closed up. Wrist should face backward and forearm should be twisted counter clock wise to generate more spinning force.
Palm centered and web space for this could generate a strong lever force and spin force. During a backhand smash, case the racket should be fully extended to the maximum angle by your arm and shoulder.
Fingers should be tightly gripped together also.