All joking aside (well, not all joking), I'll take it uppon myself to describe that a ThighMaster is.
The ThighMaster is little more then a glorified coil spring with elongated batwing ends. It is vaugely butterfly shapped, and constructed primarily of tubular steel. It's impossible to explain in words, so I'll employ some ASCII-Fu:
// \\ // \\
|| >>()<< ||
You place this contraption in between your thighs, slightly folded, and then squeeze your thighs together, and release, and squeeze... Kind of like you have to pee really bad.
Do about 500,000 reps of this and you, too, can have a body like this.