"Double down" comes from blackjack: you double your bet when you’re confident...

https://idiominsider.com/talking-odds-at-work-the-risk-idioms-every-english-learner-gets-slightly-wrong/

"Double down" comes from blackjack: you double your bet when you’re confident your next card will win. Many learners think it means 'try harder,' but it’s more specific—commit more to a risky move. Use it when you’re doubling your stake, not just effort.