DNA helicase that acts as an essential component of the spindle assembly checkpoint. Contributes to the mitotic checkpoint by recruiting MAD2 to kinetochores and monitoring tension on centromeric chromatin. Acts as a tension sensor that associates with catenated DNA which is stretched under tension until it is resolved during anaphase. Interacts with PLK1, which phosphorylates it. Both proteins are mutually dependent on each other for correct subcellular localization. Belongs to the SNF2/RAD54 helicase family. Note: This description may include information from UniProtKB.