This is a condition in which the knee bends backwards, or in a hyperextended position, therefore it also known as hyperextension of the knee.
Normal active (by own) ROM for knee extension is 0 degrees. Passive knee extension up to 10 degrees is normal.
In this condition the knee moves into hyperextension, i.e. beyond the normal knee extended position of 0 degrees. This puts extra strain on your knees which can be very harmful to your knees.