You should first learn how to match revs without braking, to learn exactly how much you have to depress the throttle pedal to match the revs on a lower gear. When you're able to do that well and have developed the proper feeling, you can start doing it under braking. Be sure to do it when there's not much traffic around and always give priority to the brake pedal, because when you're starting to learn H & T your right foot can slip of the brake pedal the moment when you're blipping the throttle, which is obviously very dangerous...
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