Hi. I recently started running this game, but i can't really tell what determines when granpa starts to spin. Is it just random when you "dash", or is this some secret input? ;)

You hold up, then you transition from holding up to tapping down

Then as you've seen already after a few spins he has a cool down period where stars appear around his head, because he's dizzy, and you have to wait for a bit until you can use the spin again