A hyperextension injury is one caused by excessive hyperextension (knee bending backwards).
Hyperextension injuries of the knee: do patterns of bone bruising predict soft tissue injury? Ali AM, Pillai JK, Gulati V, Gibbons CER, Roberton BJ. Skeletal Radiol. 2018 Feb;47(2):173-179. doi: 10.1007/s00256-017-2754-y. Epub 2017 Aug 31.