As long as you have 2 soups or fewer, your melee hits deal +30 % damage. Nothing to activate, nothing to recharge: your own state decides, permanently.
It is the only kit that rewards not healing. The cost is total — on a soup server, playing nearly dry is playing without a net, and a mistake cannot be repaired. The 30 % does not make up for the lost healing: it makes up for the risk of no longer being allowed to be wrong.
A Mushin who keeps his soups is a kit with no ability. That is exactly the intent: the bonus is earned every second.
The counter is to see it coming. A Mushin hitting hard is a Mushin with nothing in reserve: you only have to hold. It is the only kit on the server where the right answer is sometimes to back off and wait.