It's caused by blood pooling in the legs, so there's insufficient return to the heart and brain. Normally, moving one's legs helps to pump blood back to the heart through muscle contraction. Locking the knees makes standing still easier, but reduces use of the leg muscles. In some people, this pooling may lead to hypoperfusion of the brain and fainting, which is essentially the body's way of putting the brain and the legs and the heart at the same level.