I understand 'buckle up' and I can see the move people act: pull up seat belt and fasten it. And to 'buckle down' I only know that it is to start working very hard and don't know why. Could you please explain the move 'buckle down' to me?
Please give us a sentence that uses "buckle down" in the sense you mean, diegoclegane. I take it you're not asking about the "start working very hard" meaning. Where have you seen it used in another sense?