A flex rocker strips away unnecessary weight and structure.
The rocker is the component that swivels on the ground board and allows the telescope tube assembly to swing up and down. The Teflon altitude bearing pads are on the sides of the rocker and the azimuth bearing pads are on the ground board and face upward. These azimuth pads rub against the rocker bottom board, covered with Formica.
As telescopes have become lighter with the ultra-light movement (began by Steven Overholt), their center of gravity has moved closer to the mirror box and the altitude rims have become larger. This results in a rocker that is not very tall. Very short height rockers can be too flexible.
A flex rocker turns this weakness into an asset by placing the azimuth bearing pads directly underneath the altitude pads. In extreme cases, the altitude and azimuth bearings can be made from a single piece of Teflon rod or square stock. Commonly the altitude and azimuth pads are separated by a short piece of wood or aluminum, called a ‘post’. The four posts need only be kept in position by lightweight thin spacer rods.
Here is a Google Sketchup of my ZipDobII, Joe Rotman's 16 inch [41cm] UltraLight, Greg Babcock's circular flex rocker 24 inch [61cm] Ultralight and the first flex rocker I know of, Dan Gray's 14 inch [36cm].
The bending force on a post due to pushing the scope is:
Bending force = push needed to break static friction * altitude bearing radius / ratio of upper end to center of mass / 4
Bending force = 1.5 lbs * 23 / 12 / 4 = 0.7 lbs
These forces are very small, which is why thin spacer rods between posts is sufficient.
In order to keep the post height to a minimum, it is common for the azimuth bearings to sit on a rigid center-less base ring. This allows the mirror box to swing through the base. The eyepiece height above the ground when the scope is pointing upward is minimized when the mirror box sits inside the base ring just above the ground.About the same time as Dan Gray was building his flex rocker, John Sherman was independently inventing the flex rocker with slightly taller rocker sides, see http://www.john-pix.com/a/flex/index.html