The ITF Technical Commission determined the Retenser conforms to the Rules of Tennis Appendix II.
APPENDIX II THE RACKET
For all measurements in Appendix II, SI units shall take precedence. a. The racket shall consist of a frame and string(s). The frame shall consist of a handle and head, and may also include a throat. The head is defined as that part of the racket to which the string(s) connect. The handle is defined as that part of the racket connected to the head which is held by the player in normal use. The throat, where present, is that part of the racket that joins the handle to the head. b. The hitting surface, defined as the main area of the stringing pattern bordered by the points of entry of the strings into the head or points of contact of the strings with the head, whichever is the smaller, shall be flat and consist of a pattern of crossed strings, which shall be alternately interlaced or bonded where they cross. The stringing pattern must be generally uniform and, in particular, not less dense in the centre than in any other area. The racket shall be designed and strung such that the playing characteristics are identical on both faces. c. The racket shall not exceed 73.7 cm (29.0 inches) in overall length, and 31.7 cm (12.5 inches) in overall width. The hitting surface shall not exceed 39.4 cm (15.5 inches) in overall length, when measured parallel to the longitudinal axis of the handle, and 29.2 cm (11.5 inches) in overall width, when measured perpendicular to the longitudinal axis of the handle. d. The racket shall be free of any attached object, protrusion or device which makes it possible to change materially the shape of the racket, or its moment of inertia about any principal axis, or to change any physical property which may affect the performance of the racket during the playing of a point. Attached objects, protrusions and devices that are approved as Player Analysis Technology, or that are utilised to limit or prevent wear and tear or vibration or, for the frame only, to distribute weight, are permitted. All permissible objects, protrusions and devices must be reasonable in size and placement for their respective purpose(s). No energy source that in any way could change or affect the playing characteristics of a racket may be built into or attached to a racket.