Q: How much does bluffing affect the game at the Pro Tour or Grand Prix level? Do you intentionally bluff your opponent a lot, or do you tend to play it like it is and leave your mana as available as you can signaling some possibility? What's the best bluff you've ever seen/performed/had performed on you?
A: There are two main types of bluffing, the attack phase bluffing and the cards in hand. I usually don't think bluffing is such a great idea while attacking, except for two cases.
1) You're losing the game no matter what. It is locked and you have no options left or your opponent put a clock on you. When playing perfectly is not even enough to win, you have to figure out plays that help lowering your opponent's life total.
2) You have the conviction that there would be no point for your opponent to block. For instance, attacking with a Veteran Armorer into the Viashino Fangtail your opponent just played. If he waits for one turn, he can trade Fangtail for Armorer + Wildsize or Gather Courage so he will probably just take two damage.
Attacking into a 4/4 would be way less good for he could block and make a two for one in the worst case. He can still think of Orzhov Euthanist and Gaze of the Gorgon, but he will probably take the risk if he hasn't seen any of these cards in either of the two games.
Bluffing is risky, but it can become an advantage in this situation. If the opponent doesn't block, he'll play all the game around the card you are supposed to have in hand. Keep it in mind.
The other part of bluffing consists of trying to make your opponent believe you have something in hand. For instance you have Scatter the seeds in hand and you want your opponent to attack into it. If it's your only card in hand, you can advance it to your land pile and finally decide to keep it in hand. When your opponent considers attacking, grab your pen just like you would write damage ...
The complaint bluff can work pretty well too, pretending you are so unlucky you drew a land again hoping your opponent runs into Devouring Light or Dismiss. I usually don't do it for I consider it a lack of respect to the opponent.
I forget about most of my games pretty fast, so it's hard to pick one in particular – maybe during my first Euros in Berlin seven years ago. I am playing black Suicide and my opponent is playing blue-green Control. He's tapped out, and his only untapped creature is Spike Feeder. I've a Sarcomancy token and a Cursed Scroll I can activate at the end of turn.
Having three different cards in hand (Dark Ritual, Hatred, Wasteland), I have not many chances to kill his Feeder, but if I can have him, even if he sacrifices I'll win on next turn playing Ritual and Hatred on my guy. If the game gets one turn longer, it will become a lot more difficult for me. So when he passes I activate my Scroll as fast as possible on his 2/2 naming Swamp. Assuming I probably have two or three in hand, my opponent sacrifices it and I win the game on next turn casting lethal Hatred.
Looking back to this game, it doesn't seem like an amazing play, but I remember feeling very happy about it then.