The knee - range of motion | KNEEguru notebook
This website explains the mechanics of the screw home element of knee extension...
also I forgot to mention, relating to the question about how hyperextension can cause ACL injury. as the MCL and LCL also restrict hyper-extension, if there is laxity here then the ACl is brought into play for a role in preventing hyper-extension. Normally it will be combined with a rotational or valgus force, but in this sense the ACl can be damaged in hyperextension