hey there, remember as already stated that the acl is not one bundle of fibres. it is more accurate to describe it as a ligament that is composed of three bundles of fibres that each are taught in different positions. one of these bundles of fibres is indeed taught in full knee extension. with a forceful hyperextension its therefore possible to tear this bundle. some people think that with a hyperextension injury, this part of the ligament is torn and it then wont provide the normal stability and can easily be finished off with a typical functional valgus position injury. an interesting video to look at is michael owens acl tear in the 2006 world cup. he stated that he felt a tear and heard a pop before his foot hits the ground, so its possible to say that he may have torn part of his acl before his foot comes down, he goes into a valgus position and blows the whole acl out. there area a couple of acl tear videos out there like that and its interesting to look at the mechanics of the injuries in slow mo.

some info on the anatomical bundles. http://www.ncbi.nlm.nih.gov/pubmed/21695467