Required for high-fidelity chromosome segregation during the later part of each cell cycle. Acts in opposition to the phosphatase PP1. Has a role in attaching the kinetochores to the microtubules and ensuring that sister kinetochores connect to opposite poles. The promotion of bi-orientation is achieved by selectively detaching kinetochore-microtubule attachments that are not under tension (By similarity). {ECO:0000250}.