Excerpted from Robert's Rules at
www.homepokertourney.com.
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22. Showing cards from a live hand during the action injures the rights of other players still competing in an event, who wish to see contestants eliminated. A player in a multihanded pot may not show any cards during a deal. Heads-up, a player may not show any cards unless the event has only two remaining players, or is winner-take-all. If a player deliberately shows a card, the player may be penalized (but his hand will not be ruled dead). Verbally stating one’s hand during the play may be penalized.
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If he does this heads up, I say ok because he's not gaining any advantage but putting himself at a severe disadvantage if he's not all-in. Think about it, guy flips his cards before the first to act has done anything, that guy will either check/fold or bet now. If someone did this to me and didn't have the nuts, I either bet big or go all-in; pretty tough to call in this situation when I bet knowing his cards.