PCL = Posterior Cruciate Ligament. Most people make a big deal about the Anterior Cruciate Ligament (ACL) but the PCL is just as important obviously. Basically there are two ligaments that connect your femur (thighbone) to your tibia (shinbone).. one front (ACL) and one rear (PCL). The PCL keeps your shinbone from moving too far backwards past the thighbone. I suppose too much twisting would hurt both of them. I looked at some info and most PCL tears/injuries are caused by smacking the front of the shinbone on something hard with the toes pointing downwards (like falling on your knees really hard) which pushes the shin backwards and tears the ligament. Also, it appears that having strong quads can take some stress off the PCL and cause less pain. I would assume also that having strong hamstrings would take stress off the ACL as well. I've recently gotten back into cycling and when I get leg pain, it's not in my knees. I actually get pain in my lower quads above the knees, but it's a definite muscular pain like you would get in your biceps after heavy lifting.

edit.. just found some further info here: http://www.hughston.com/hha/a.pcl.htm or for those of you that can read bigtime medical mumbo-jumbo, here's a really informative one.. http://www.emedicine.com/pmr/topic102.htm