Right click the lead: you chain a nearby player for 6 seconds. If either of you goes further than 5 blocks, you are both pulled toward the middle.
It ties both ends, and that is the balance of the kit. Chaining someone stronger than you is signing for it: you cannot break away either. The Kusari does not say "I will catch you", it says "we settle this now" — and that is only a good idea if you are winning.
Nothing happens while you are within five blocks: you fight normally. The chain only tightens beyond that, and in proportion to the gap — one step too far barely corrects, an outright escape costs dearly.
An action bar permanently shows both players who is chained, the distance and the time left.