The rear wheel uses a sprocket carrier mated to the hub. Between the carrier and the hub a series of rubber blocks resides. The so called Cush Drive. The slop refers to the clearance between the blocks of rubber and each of either the sprocket carrier or/and the rear hub.
Grasping the rear sprocket and trying to rotate it forward or aft, will give you an idea of the clearance.
Various factors* may cause this, but the overriding cause seems to be the cush drive rubbers. The OEM ones are ever so slightly too small. Members have found that by ordering and installing the Thunderbird cush drive rubbers the issue goes away. The Tbird part # is T2012211.
*Other causes are loose chains, worn out chains/sprockets.
