Can someone explain?

[Rottersod]

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most people are clueless what to do out of position, but im really amazed by his play, im assuming this is some kinda freeroll or 0,10$ tournament or you got really really lucky with your table.Haven't seen this kinda plays even in rebuys.

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Why are you confused about SB's play? It seems like a perfectly good play from a suited 43 in the SB if he had a plan to push a flop that wasn't coordinated. He knows that if MP's had a really good hand they would have come over the top pre-flop and he's just putting you on the kind of hand you have. Fold. No reason to think your AK is all that strong here.