There's another one which, while a bit more brutal, works wonders...
Lie on the floor, on your back, and hook your legs over a broomstick placed across two dining room chairs, so that the stick is under your knees. Let your feet and ankles go free.
Shuffle around, until you realise that your lower back disks are in fact off the floor, and you'll have then opened up the disks to their full extremity!
(I did this in our Maidstone office once, and our lovely secretary wandered in, just as I realised I'd loosened the trousers and help make the pain just go away..;0~ )
Sometimes it takes just a few minutes to explain, but as she was so gorgeous, I seem to remember that I took about half an hour...