Putting the term into Google gleaned the following Wikipedia article:

http://en.wikipedia.org/wiki/Steering

About halfway down the page you will find a tiny paragraph stating:

"Some vehicles now offer a form of "passive" four-wheel steering, where the
bushings by which the rear suspension attaches to the automobile are
designed to compress in a precise direction under the forces of steering,
thus slightly altering the rear suspension geometry in such a manner as to
enhance stability."

Another article, seemingly more in-depth:

http://www.ozebiz.com.au/racetech/theory/align.html

And again, about halfway through:

"It should be noted that in recent years, designers have been using bushing
compliance in street cars to their advantage. To maximize transient
response, it is desirable to use a little toe-in at the rear to hasten the
generation of slip angles and thus cornering forces in the rear tires. By
allowing a bit of compliance in the front lateral links of an A-arm type
suspension, the rear axle will toe-in when the car enters a hard corner; on
a straightaway where no cornering loads are present, the bushings remain
undistorted and allow the toe to be set to an angle that enhances tire wear
and stability characteristics. Such a design is a type of passive four-wheel
steering system."

The two articles correlate well enough for me.  (-;

~Brian

Alfa Romeos have used rear-axle steering for many decades.
My 1969 GTV has a live axle, located fore-aft by trailing arms
and laterally by a trunnion arm having a compliant bushing
directly above the diff, is such a case. As the car rolls past
about 6 degrees, the inner end of the axle is drawn forwards
by the trailing arm, inducing rear steering which tends to
prevent early loss of grip.

The action of the legendary Alfetta (115 series) De-Dion rear
end is even better, because the lateral location is done by a
Watts linkage, carefully calculated to keep the axle almost
dead straight until you reach about 8 degrees of roll, then to
progressively (by more rapidly than in my GTV) to bring in the
same kind of rear steering. As far as I know, this general
form of rear end had been around for decades even then.

Both cars are amazingly forgiving yet generated unequalled levels
of grip for their times - just a joy to drive. (Neither has the
rear toe-in mentioned by Brian though.)