Evidently the device does actually work. SawStop says that, during
development, testing, and public demos, they have triggered the brake
over a thousand times. One could discount that testimony as a
magician endorsing their own parlor tricks. Maybe the SawStop
employee doing each demo nudged a switch with his foot as he was
feeding the plywood and hotdog through the blade. I guess that's why
Fine Woodworking ran their tests - to make sure that SawStop wasn't
tricking people. FWW concluded that the device is real and it
works.

The device is fairly simple. Once you understand it, you might wonder
why it hadn't been thought of sooner. They electrically isolate the
blade, then feed a low voltage signal to it. They monitor the signal
(about 3 volts). When the blade comes into contact with an object
that has a lot of electrical capacitance, such as a human body, the
signal voltage temporarily dips. This dip is much more pronounced
when contacting a person than when cutting wood - unless you commonly
rip pieces of wood that weigh 150 pounds, are at 80% moisture content,
and have large unbroken electrical pathways throughout similar to our
blood vessels.

Once contact is sensed the brake cartridge sends an electrical pulse
through a stainless steel wire that is holding a spring compressed.
The released spring pushes an aluminum brake pawl into the backside
of the spinning blade. The blade burys itself in the brake pawl.
Much of its rotational momentum is transferred into downward force,
much like a car with its motor racing being knocked off concrete
blocks, and the blade drops below the table top. This dropping
motion is allowed by a specially designed arbor block with a pivot
and spring loaded retainer.

To put the saw back into service you work the joined blade/pawl off
the arbor and mounting posts, install a new blade and brake
cartridge, and snap the arbor block back up into position. There is
no damage to the saw. SawStop says that many of their tests have
been on a single saw, and it is still running normally and in
adjustment.

The cabinet saw already released includes hardware to implement this
concept that may be overengineered and too carefully built. They are
very interested in having the device work properly. It will be
interesting, though morbid, to hear the first reports of actual
contact with operators in regular shop situations.

Hope This Helps,

Dave