Hi,

The spin test you are talking about would ensure that the wheel bearings are not to tight, by the backlash once the wheel stops(generally only used as a final check when adjusting wheel bearings). To check the play(looseness) in the bearings(which would cause a horrible vibration through the car once at high speeds, somewhere over 70km depending on how loose the bearing is), you need to jack the rear up the same but push the wheel in towards the diff and back out towards yourself, do this continuously with pressure about 3 times. If there is play you will feel the wheel move slightly between the bearings..