Using mitts, heavy bag or makiwara.
Hitting the bag, the mitts or the makiwara, should be done at least 3,4 times a week for 10-20 minutes, it is a great and necessary way to develop your kime and impact power and to get feedback on your quality of movement.
There are few things to consider:
First be careful of tensing too soon, that will kill your speed, kind of like driving with the brakes on. We need maximum speed and momentum than pressure/contraction at impact. When I hit the bag or makiwara I let the contraction happen as I hit the bag, we need to reach the target at maximum speed, undue tension before contact will slow us down.
I let the bag decelerate my punch, which results in ecentric contraction, which is the muscles are lengthening while contraction, muscles are working to direction of technique.
Second, Sensei Nishiyama gave me very useful tip when hitting the makiwara, at impact I want to feel the reaction power coming back to my center (low stomach), rather than to the elbow or shoulders. That is a good feedback, that will tell me if I am aligned properly and/or if the contraction sequence is optimal, and if the technique was initiated properly from the body center. It also tells me that the whole body momentum is meeting the target.
Third, I like to think of pouring the energy from my body through the fist to the bag or mitt. This is helpful, because some people tense in a way that braces and “chokes” the energy, and energy is absorbed into our own body, this should be avoided.
Fourth, don’t push, it is not about muscular power, push is power delivered over long period and is ineffective. Use total body snap, your body should be like a whip from the center out. We need to teach the nervous system to recruit more motor units in shortest time.
Fifth, when you hit, nothing moves externally for a moment, yet inside momentum does not stop, kind of like a car crash, the sharper and more at once the car stops, the more momentum will the passengers inside will receive. Use the mitt or bag to understand how long does it take to transfer the full momentum to target, because kime should not be any longer or shorter than that, once energy is transferred the reaction should be Zanshin with the next breath.
Sixth, at contact have a strong stance, to deliver the energy from. Remember, you cannot shoot a cannon from a canoe. At the same token, you should also be able to deliver power of one leg, even though it is not optimal it is sometimes necessary so I spend some time hitting from one leg stance, especially during combination techniques.
Seventh, develop single techniques power first and than combination, you should be able to make 2,3 or 4 techniques with full speed and kime and completely relax in between. If one does not relax instantly from kime, the following action is likely to be stiff, pushing and lacking snap. Use exhalation to relax and use the energy from one kime to next action.
Avoid the temptation to use top heavy power, don’t go after end results and muscle the techniques, make sure each action is from feet and ground reaction, if you cannot do it at first, slow down, coordination is more important than power at first. When moving from the feet it will be easier to relax the top muscles.