Required for assembly of the mitotic spindle. Interacts with spindle microtubules to produce an outwardly directed force acting upon the poles. Following spindle assembly; CIN8 and KIP1 apparently act to oppose a force that draws separated poles back together. This force seems to be mediated by KAR3. {ECO:0000269|PubMed:1618897; ECO:0000269|PubMed:1643659}.