I believe you are still too tense in your arm and wrist during the swing. The wrist only comes into play at the last moment. You are also losing alot of power by not using your body rotation enough. The upperbody/shoulder rotation comes first, then the arm follows and at last comes the wrist. To get a feeling for it, you could only do the body and shoulder rotation while keeping the arm and wrist completely loose. If you do it right, the racket should do a circular movement behind you back (maybe you can practice it infront of a mirror?). Inthe video that BadBAdmintonPlayer has linked on page 1, at 21:55 YYS shows it perfectly. At 21:55 you can see his hip rotate, then his shoulder, the elbow comes foreward, at 22:04 his arm rotation starts and only at the last moment the wrist tenses up.