Best Strategy When Down Material?
I just keep myself calm. Don't be frightened of their attack. Just try to carefully defend and try to find useful combinations which can bring you back in the game. I remember once I won when my bishop completely outplayed my opponent's rook in an endgame. Try to find more and tactical plays and all the best for your next games!
In general, it is wise to keep the pieces intact and avoid exchanges as you would likely need these pieces to coordinate a planned counter-attack. The more we exchange pieces, the more it benefits the winning player as the chance of a counter-attack reduces with fewer pieces on board.
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For me, when I am on the winning end of the position, I will look out for opportunities to simplify the game into an easily winning game.
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Do note that this is just a general rule of thumb and it may not apply to all contexts.