Hyperextension is when the knee can bend a bit backwards.

Knee range of motion, showing that the normal knee may slightly hyperextend - in this case to -6 degrees.
Range of motion of the knee
A normal range of motion at the knee includes flexion, extension and a small amount of hyperextension.
-
-
CLINICAL PUBLICATIONS
Cruciate injury patterns in knee hyperextension: a cadaveric model. Schenck RC Jr, Kovach IS, Agarwal A, Brummett R, Ward RA, Lanctot D and Athanasiou KA. Arthroscopy. 1999 Jul-Aug;15(5):489-95.