Instead of have holes in the disc where it mounts on to the bell, it has slots, where oval shaped bobbins are inserted (with bolts running through these). The bobbins are slightly smaller than the slot in the disc, which enables the disc a certain freedom of movement (hence "floating"). This is essential with such large discs, as the expansion and contraction would cause the bolts to shear. Also the leverage effect of applying the brakes would put a high load on the bearings and cause pad "knock off" (where the pistons are forced back into the calipers under hard cornering). This freedom of movement prevents this.