I know what you mean, I 'THINK' it may be refered to as backlash... basically, it is when the diff turnes, there is play between the pin in the dogbone, and the 'cup' wall, this slack is taken up and will 'flick' (For lack of a better word) the dogbone into motion and the dogbone will take up the slack in the same way on the wheel side... this happenes a number of times before the car begins moving, and then happens when you brake... when the drivetrain wears, this gets worse... I hope this makes sense to you all....