You lie on it in all different positions and roll backwards and forwards on it, and when you find a particularly painful spot, you rock back and forwards on that spot.
Those lumps are designed to inflict even more pain.
I'm reliably informed that it is meant to hurt a lot.
It has worked though, my hip/knee feels a lot better.
Unfortunately, I've been hobbling around so much that my ankle is a mess now.
I'm going to try going for an easy bike ride soon as I'm bored of being injured.