When assembling a kendama, after inserting the string in the tama a bead or bearing is put on the string before tying the knot to hold it all together. Originally this was just a plastic bead which was simply there to hold everything in place, however as kendama has evolved people and companies have taken to the metal micro bearing instead. 

It helps with suppressing potential tangling of the string during long combos. Under normal circumstances when a tama or ken spins the string would spin with it as it’s “locked” to both of the pieces, a bearing aims to alleviate this, primarily on the tama spinning end. When a tama with a bearing spins the idea of having a bearing is that due to the unique construction the bearing can “absorb” the spin from the tama, without it affecting the string. The bearing is made up of two seperate rings that can spin independently of each other, allowing the string to remain completely or partially tangle free when it could otherwise have posed an issue to an elongated combo.