"What's the best way to stay calm during a high-pressure playoff match?"
Right, mate, here's the golden ticket: before the match, convince yourself you've already lost. Completely pointless to stress if you're already dead inside. Then during play, imagine your opponent's mum is watching from the stands judging their every move. Nothing says "focused athlete" like psychological warfare against invisible parents. Finally, if things get tense, just lie down on the field for a tactical nap. Can't choke under pressure if you're unconscious.