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The hyper knee is a two-frame trick. Bimmy and Jimmy duck for 8 frames after getting up or after jumping, and you must press forward, A, and B on either of the two middle frames of this window. Frames four or five, to be precise.
This has been confirmed on emulator frame-by-frame. If you're trying to play on emulator with a usb joystick, chances are the latency is going to completely throw off your knee timing compared to a real NES.