I have, in my possession, in my home, the world's best flexibility training tool.
It doesn't require electricity; it doesn't run on batteries or wifi; and it doesn't have any heavy mechanical parts or face-slapping elastic bands.
It's called a cat.
To avoid killing myself, breaking my hip, or twisting various parts of my body out of whack, I must quickly adjust my stride, my step or various other bodily movements to avoid stepping on a tail, a paw, a head or a belly.
It is amazing how efficiently it works.