Some of the Sawstop patents had claims concerning the blade dropping below the table. There have been saws available for decades, that use a circular blade of the type used on a tablesaw, where the blade can be raised above the saw table to cut, and when a handle is released, the blade will automatically drop below the table, either due to a spring, or gravity, or both. The main saw of this design I know of is called a Gjerde saw after the inventor, and is commonly used in Scandinavian countries. I believe there are other designs as well though.
Another of the Sawstop patent claims had to due with the sawblade lowering due to the centrifugal force or momentun of the spinning blade. It would not surprise me if past saw designs, had cutting retractable cutting blades, whose retraction was assisted by the centrifugal force of a spinning blade, even if no patent had specifically been issued related to this. If there was a saw design from the past that could be shown to have this feature, it would count as prior art, and this patent claim would not be considered valid.
I haven't been able to find any more information on the subject, but when the Sawstop patents started to be discussed at length on various forums and magazines, I saw a comment from somebody who claimed that dyring the 1960's or 1970's, there was a tablesaw of some sort shown at a woodworking or industrial show in Germany, whose blade was designed to stop in case the blade contacted the user. Supposedly the woodworkers who saw the saw laughed at it, and nothing more came if it. Unfortunately, I don't speak or read German, so I haven't gone to the effort of trying to see if a German patent was issued for the saw. From the way the claims in the sawstop patents I've read are written, I suspect that there was some sort of prior art that would have prevented a patent from being issued, for the blade stopping simply because of physical contact with the blade.