Your v-sync cable keeps all those frames rendered by your video card in sync with your LCD display. Of course, it's much more complicated than that, but if you're getting some screen tearing, this might be the culprit.
593-1241 922-9481 Vertical Sync Cable for A1312 27inch iMac Mid 2010 NB-2779W This is a replacement v-sync LCD cable for the 27" iMac released in mid 2010. Apple part number: 922-9481, 593-1241