In Susan Polgar's book "Chess Tactics for Champions is one of the following:However, I have found that Black can defend by 6. ...Nc6 7. Qxc6+, Bd7, the Black rook protected by the queen. Is this true? Black only loses a knight, not a rook. Will someone please tell me whether my analysis is right? It would be nice to have spotted an error which a grandmaster missed.

Ok, the moves were 1.d4, d5 2.c4, dxc4 3.e3, b5 4.a4, c6 5.axb5, cxb5 6.Qf3 Polgar claims the rook is trapped.

Yes, that is correct, black only loses a knight.

However, Polgar only says the rook is trapped, which is true, you're just sacrificing a knight to free it.

That's true, but she should have said that it costs a knight to free it, because she gives the impression that it is lost.

It's still a winning move.

Yes, but it's only winning, it doesn't make it won.

It does at high levels of play. 

Yes, but theoretically, if traded down from there, an extra knight can't win. An extra rook will.

It's a big assumption to think that black could exchange off all the material from there.  What's most likely to happen is that white's extra piece will net further material gains which will, in turn, result in a win.

Even if white gained the rook, there is no guarantee that white will win.   But with accurate play, white will win with the extra knight.

He's got you there!

Yes, but this is theopretical. If your up a rook, you just trade down from there and you're safe. If you're up a knight, trade down from there and you're stuck.

If you're up a whole piece then the game is won barring gross errors.

Your theoretical scenario involves all material, including pawns, being liquidated. It's true that that's not winning but that situation is pretty uncommon isn't it? At the very least you're going to have pawns + extra piece vs pawns. Seems pretty winning to me.

You have to think about it this way- up a minor piece, at some point you won't be able to safely trade down, or IMM. Then, you're opponent is going to bug you with trades until you go crazy. You probably will win anyways, but it is a lot more annoying. With a rook, it's easy, because your opponent can't risk trading down, because rook can mate.

Let's play a game.  Give me a knight and see how you go at trading down to the draw.

What is that supposed to mean?

They're both winning. One is an easier kind of winning, or, if you like, more winning.

The end. Lets all baste ourselves in spices and turn in for the night.

P.S yes, sambirder, take up Borg on the offer. Give up a knight early on and then trade down for a draw. That'll be awesome.

what do you think it means?

Simply put, the extra knight so early WINS if white plays well.

As I said, with accurate play, white will win.  Obviously not if black is a GM and white is an average club player.