My explanation/understanding is that fast walking with larger strides creates rotational movement around the spine axis and that stimulates the tissue. With slow walking, one could do that with no body rotation and only the legs move.
I'll experiment with it and try to isolate it. The description reminds me of twisting but twisting is often associated with more pain. However there are some videos that suggest similar movements relieve symptoms. This one[1] seems quite popular.
