Somax really developed this idea as a training device for golf.
The hip rotation in golf, tennis, swinging a bat and pitching are not very different. The hips are generally undertrained I think. There are several devices (belts using bands) as hip trainers on the market. The first one I am aware of (this may not be accurate) was Dr. Michael Yesis in 1999 with his excellent book Explosive Golf. He sold a hip belt with resistance bands to help teach proper hip lead, separation and rotation.
The Somax could provide some benefit I am sure…10 MPH in 12 minutes is clearly a cheesy, unrealistic sales pitch.
Lantz Wheeler (who posts on LTP) has a belt he is selling to help develop velocity and body control…do some research, see which makes the most sense in terms of potential benefit and cost.
In the Somax video it appears the belt is more around his waist than his hips…the rotation etc should really be from the hips.