The kenchi-ku are a sub tribe of the Hai'bu tribe. They are smaller than members of the Tensai tribe and used to aid the Hai'bu in their work. They would be kicked into places where the Hai'bu couldn't reach (as they are bigger than bots in other tribes on average) to clean those places. The Hai'bu started kicking them about for pleasure and the game of BotBall was invented. Their current job is literally being a soccer ball (I am not making any of this up, this is from the show) with most bots completely unaware that their balls are bots.
BotBall is a spectator sport in Flip City that is like real world soccer. The field is made up of two levels with holes doted about the place where the bots go onto the second level below the pitch. There appear to be two holes at either end of the field placed high up on the upper layer. The goal like in soccer appears to be to get the Kenchi-ku into that hole to score a point. The bots are allowed to transform into their ball form and the game also appears to be a contact sport. Hand contact with the ball appears to be allowed in the game.
The game has high popularity and can lead to riots just like really world soccer as evidence when star BotBall player Roboto was kidnapped and the game ended due to the kenchi-ku rebellion/strike/disappearance (they did rebel, and seem to have preferred to clean up trash than being soccer balls).