Big Think?

All,

Have started watching some GM matches with commentary and one item that is interesting is that a couple times during the match one of the opponents will stop and think for what seems like a very long time.  They don't move and I certainly hope they are still breathing but it's hard to tell.  Might have been our own David P. who called this a "Big Think".  So what are they doing in general?  Planning different lines to play, risk reward, etc.  How far can they calculate out?

Thanks all, just thought to open it up for any insight. 

D

My experience: The first big think they sometimes have is when they deviate from opening preparation (around move 30 or so ) and have to come up with a plan. Other times they realize they made a blunder and are having a self-beat session. Aside from that they probably think, or sense, the position is critical and are calculating very carefully.

^ Yeah.  They're either trying to decide what to do with the position (i.e. what's going on) or it's a critical position and they're trying to find a win / avoid a loss.

Of course just guessing because I'm not a GM myself :p