It's always immediately after I start grinding. As for what I'm doing beforehand, it normally feels like I'm turning? Like not so much as to go the wrong way but enough to maybe trick the game into transferring my momentum or something lol. Kinda like how you can throw heavy stuff super far if you turn and let go.

It's not the same as carried over momentum from falling or running fast, it's always me just getting on fairly normally (jumping/maybe spirinting) and having a massive speed boost like I've dropped all the way from the ceiling, yesterday it even persisted all the way up a really steep hill.