ONE MOVE CHANGED THE GAME

In chess 1 quickly played move can change the whole game.
In this game i am white and the time contol is 20 minutes each.
During the game i did not play well at all,i could not get my thinking
head going.
Then 1 very quickly played move by my opponent Mike changed the game.
His quickly played move was 31....Nf5,when he played this move he still had about 12 minutes on his clock,so being short of time was not the reason.
After the game Mike said he was thinking i would play 31.Qe2 then double my rooks on the h file.
So when i played 31.Qb1+ it sort of took him by surprise a little,and he just moved without thinking. Then it was an up hill struggle as he only has a pawn for the knight.
His passed pawn is not a threat at all.

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