After the first Sniper Wolf fight, Snake gets captured, and Ocelot straps him to a table and runs electricity through him. Your life bar drains while it happens, and you keep it up by hammering the action button as fast as you can.

There's also a button to give up. If you give up, the torture stops, but Ocelot takes it out on Meryl, who's being held somewhere else on the base.

This is the big choice in the game, and it decides which ending you get:

ENDURE IT (don't give up, even once): Meryl lives.
GIVE UP: Meryl doesn't make it.

HOW TO SURVIVE IT: There's more than one round of it. Set the controller on your leg or the table so you can hit the button with more than one finger, and don't stop until the screen says you can. On Extreme my thumb hurt for a day. Worth it.

After that you're in a cell. One way out: Otacon slips you a bottle of ketchup. Lie down on the floor of the cell with the ketchup equipped, and the guard will think you're bleeding and come in to check on you. It's ridiculous. I love it.

MERYL ENDING
If you endured the torture, Meryl is alive at the end, and she's the one with you in the jeep during the escape. The last scene is the two of them riding away together in the snow. It's the "good" ending, if you want to call it that.
YOU GET: the bandana. Start a new game on the same save after this ending and Snake has a bandana in his inventory. Equip it and you have unlimited ammo. Every gun, every grenade, forever.

OTACON ENDING
If you gave up during the torture, Meryl is gone, and Otacon is the one riding with you at the end. It's sadder, and in some ways I think it's the better scene. Otacon has some things to say about the people on that base that stuck with me.
YOU GET: the stealth camouflage. Start a new game after this ending and you have the same invisibility suit the Ninja wears. Equip it and the guards and the cameras can't see you. It makes the game a whole different kind of easy. (It doesn't make you bulletproof, and it doesn't hide you from everybody. Find out who.)

MY ADVICE: Get both. Play it once enduring the torture and once giving up. The two endings tell you different things about the story, and you'll want both items for later runs anyway. I did the Meryl ending first, which I think is how it was meant to go.

AND STAY THROUGH THE CREDITS. Both endings. There's a phone call after them. That's all I'm going to say, even on the spoiler page.

Yes, it's real. Finish the game twice on the same save file, and on your third time through, Snake infiltrates Shadow Moses in a tuxedo. Every cutscene. Every boss. Black tie, in the Alaskan snow. He looks extremely serious about it.

This is the reason my first two runs were on the same save. My third run, the first one in the tux, was on Hard. Psycho Mantis in a fight with a man in a tuxedo is the best thing I've seen this year, and I saw The Phantom Menace three times.

Run 5 was a fresh save, no bandana, no stealth, no tux, no excuses. I got through the torture room on Extreme without giving up once, and I'm prouder of that than of my diploma. Don't tell my mom.