"Runout" The shaft should turn with no wobble...like a pencil does in your fingers.  Any small "runout" is magnified to anything attached to it....the star holding the blade.  So the blade of slightly bend spindle shaft (hit something?), or a worn or loose bearing will transmit imbalance from the rpm of the unbalance blade through the deck. This is what happens when you run an unbalanced car wheel.  A little problem becomes pounds of force felt through the floor and suspension.  You need a way of seeing if the shaft is "true."  The wire pointer is poor man's dial indicator.   "Dial indicator"...a little gage with a contact point and a pointer showing movement of the shaft, if any.