Fischer vs. Tal 1959, 20.

Hey guys, 

First I appolgize if this is only suppose to be for personal games, I looked (not hard I will admit) to see if professional games were appropriate here too. 

Anyways, having just started studying chess I've been trying to understand some of the "higher level" thinking behind some of the masters' moves. 

One move that I did not understand was Fischer vs. Tal 1959 move 20. 

http://www.chessgames.com/perl/chessgame?gid=1044001

Why in this situation did Fischer not take the rook? Am I being very simple minded thinking that he would have gained 2 points by taking the rook instead of the knight, and that makes it worth it? 

Any insight would be appreciated.

Thanks.

Nico

I took a quick look at it. it seemed the knight was free, where taking the rook would have lost the bishop. After another look I see the knight was not free, and the question about not taking the rook makes more sense now.