how to beat fast with your arms, and not use too much strenght?

[hl1978]

Slow is smooth, smooth is fast.

Same applies at the race track. Fast jerky inputs, which feel "fast" slow you down.

[hl1978]

While I would agree that focusing on the lower body is a key element in speeding up your cut, if you want to improve your swing, focus on "pulling with the elbows". Doing so will take the focus off your shoulders though they should remain dropped at all times, otherwise you will improperly raising the shoulders reducing power, speed, and some potential for injury. It will also lead to less tension in the forearms and grip which will slow you down for most of the swing, obviously the shibori type motion can speed it up at the end....

Note this doesnt mean tense the elbows, but you may find that you engage more of the triceps and have a bigger more relaxed swing.

[Halcyon]

The following tip probably won't be very useful for lower-ranking folks, partly because your form and tenouchi need to be at a certain level for it to work, but it might be useful for higher-ranking kenshi.

Try to put your focus/energy into the monouchi of the shinai and THROW the monouchi at the target, whether it's kote or men.