The Gambol With the Gimbal
The product today is mechanical, rather than optical. It has no glass in it, but several moving parts. It is second cousin to a gun mount, but not nearly as noisy.
The underslung gimbal mounting - also known as a Wimberley mounting after one of the many makers - is a form of suspension for specialist use in photography. Shooters who want to use long telephoto lenses and who need to follow the movement of wild animals, aircraft, racing cars, surfers, or field sports players will be interested. It is the answer to a number of their needs.
At this point let me introduce an analogy that was explained to me when I asked about the differences flying helicopters as opposed to fixed-wing aircraft. I was asked to think of a large imaginary bowl in the sky - hemispherical and slippery.
The fixed-wing aircraft rides inside this bowl. If it is up on the inside surface of the bowl...in a bank, turn, or climb - and the pilot lets go - the plane runs down the inside of the bowl and settles into a stable position on the bottom.
The helicopter rides on the outside surface of an inverted bowl. When the pilot takes his hands off the controls the helicopter falls down over the outside of the bowl. The pilot needs to fly it all the time.
The operator of a big, long, heavy telephoto lens mounted on a ball head tripod is in the same position as the helicopter pilot - the lens is either held up all the time or locked in position. It is unstable. Gravity is trying to take it off centre.
The underslung mount is generally on a tripod, but when an unwieldy lens is mounted, natural forces cradle it at the bottom of the arc of swing on the mount - it retuns to a stable position. It is much less fatiguing to watch and wait with it.
This example of the underslung gimbal is by Promaster and is a marvel of economy. It is heavy and precise enough to stand the long commercial lenses, however it has smooth damped action as it swings about. It is like operating a good gun mount.