With the ignitioon in the advanced setting, the plug fires the charge BEFORE top dead center is achieved by the piston. If far enough in front of TDC, there is not enough enertia to keep the crank rotating the correct direction, and the air fuel charge reverses the direction of the crank, hence the &quot;kick back&quot; Not a good idea on a Hart Parr. Facing the wheel, always hold the crank with your palms up, as this will help clear your hands, and you out of the way if it should do this. But if you remember to retard the timing it never will kick back. 

Hope this helps,

Kirk