within the plane of the intended ball trajectory this initial positioning of each of the tennis balls is provided by a ball receiving and positioning means here in the form of a pair of spaced apart substantially parallel ball receiving and guiding rails 31 and 32 carried by housing 14 and mounted parallel to and on opposite sides of the general plane of flexure of the belt and separated by a distance less than the diameter of the balls 12 and equal to or greater than the width of the belt to simultaneously impart spin to each of the tennis balls the opposite ends 19 and 21 of belt 13 are differentially accelerated or displaced during the tensioning of the belt with respect to the initial at rest position of the tennis ball in the particular embodiment disclosed herein the fastening of lower belt end 19 to the support means provided by housing 14 while the opposite and upper end is accelerated by end 22 of arm 16 represents one particular case of a differential displacement applied to the opposite belt ends in general any differential displacement including the particular case illustrated here will produce a certain amount of longitudinal motion of the belt at the ball to belt interface during the propulsion phase this is best seen by comparing fig2 and 3 in which it will be noted that the original point of contact or interface between ball 12a and surface 28 of belt 13 will undergo an upward displacement away from end 19 and.