I have been involved in sport all my life as a player, teacher, organizer. I have never seen or used a product that even approaches what GroovZone does. You learn in just five or six swings what you might never learn correctly. All
that is required is to relax your arm and wrist till it stops reacting, fighting the device. Takes a half dozen swings. The muscle memory is instant and permanent. GroovZone also makes you aware of the correct point of contact. This awareness induces use of the feet and knees to achieve that point of contact. Use of the larger muscles of the body is also induced.
I have found that the pronation of the wrist on the serve is learned instantly.
I cannot overstate the value of this, having tried this personally and with my students. It works for me in cricket and golf and should do so in any sport that requires muscle memory.