Anticipate his/her response(s) and then calculate what you will reply with! One of the best ways to psychologically pressure someone is to make quick moves. It intimidates them and lures them to also make moves quickly, without proper analysis. So the time in which they are thinking should be as useful to you as the time when it's your turn to play!
What to do/think when opponent is thinking?
If you use a checklist of sorts on your own moves, then you can use your opponent's thinking time to start the process. Only one piece or pawn will change position after your opponent moves (or two if there is a capture), so you can for example do simple things like count material, look for loose pieces, etc. If you still have time, as KeyurShah17 said, anticipate your opponent's most likely move(s) and analyse the resulting position(s).

Scan the board for general things, like how is the pawn structure, which pieces are good/bad so which ones would you like to exchange and which one you want to keep, see if there's any loose pieces or checks available, what would be good squares for your pieces etc. Basically everything apart from concrete moves

Think about the best way to achieve world peace and Utopian ideals, alternating with meditating to achieve the Zen state of "no mind" - the primal purity of consciousness itself, while in the back of your mind praying for serendipity in the outcome of the chess game at hand. That will cover all your bases.

Think about the best way to achieve world peace and Utopian ideals, alternating with meditating to achieve the Zen state of "no mind" - the primal purity of consciousness itself, while in the back of your mind praying for serendipity in the outcome of the chess game at hand. That will cover all your bases.
I agree. Then when you're done with that, evaluate the game and calculate your next possible move, based on the board position and your opponent's possible moves. Then go back to the beginning and repeat

I wonder if they're thinking about me.
Then I do like others have suggested; I assess the position. I don't do calculations. They are pretty pointless unless you know 110% for sure what their next move is going to be.

Usually you can narrow down to 1, 2, 3 possible good moves by your opponent. Calculate what your response will be to each. If the opponent makes a different move, go back and start over.

Usually you can narrow down to 1, 2, 3 possible good moves by your opponent. Calculate what your response will be to each. If the opponent makes a different move, go back and start over.
This is really a mismanagement of time. It is broken down like this:
Your move - Calculate lines
Your opponents move - Assess the board. Look for weakenesses on both sides, look for strategic opportunities..etc.....

Put yourself in your opponent's shoes and work out what they are trying to achieve. If you were playing as them what would you try to do in the position? Then you know what you need to cross their plans.

Usually you can narrow down to 1, 2, 3 possible good moves by your opponent. Calculate what your response will be to each. If the opponent makes a different move, go back and start over.
This is really a mismanagement of time. It is broken down like this:
Your move - Calculate lines
Your opponents move - Assess the board. Look for weakenesses on both sides, look for strategic opportunities..etc.....
yeah, that does make sense

The previous comments pretty much cover what I do. Sometimes I think about some moves I want to make, sometimes I close my eyes to refresh. If I'm ahead on time I may get up and leave the room for a minute. And I usually go to youtube as I like to listen to music when I play.

i found this very useful, i was trying to calculate every move as well
yeah. I used to calculate all the time too. When I changed to this other way, my game improved.
When you just calculate all the time it causes some serious issues for you:
- You will never or hardly ever come up with an overall strategy. Which, despite what some may believe, is just as important as tactical play and absolutely necessary to progress at chess.
- You can over focus on what may not even be the best move your opponent could make. We are all guilty of 'dream chess' (not 'hope chess') where we see lines that could be ultimately bad for our opponent before we see the the moves that could be bad for us. If you get focused on one of these, then your opponent plays something completely different and dangerous for you, then it can be quite distracting and you've just wasted a bunch of energy on a pointless persuit.
- Unless their move is forced or a logical recapture, you can not be 100% without a doubt sure what their move is going to be. So trying to calculate lines they may not even play is a waste of energy. Even if there are only 3 possilbe moves, and they make 1, you have wasted energy calculating 2 lines that were never used.
It's not an easy habit to start. But once you get it going, you will see a new dimension to your games.

it's like the general vs specific thinking.
specific when it's your move, general when it's your opponent's.
good point
i found this very useful, i was trying to calculate every move as well
yeah. I used to calculate all the time too. When I changed to this other way, my game improved.
When you just calculate all the time it causes some serious issues for you:
- You will never or hardly ever come up with an overall strategy. Which, despite what some may believe, is just as important as tactical play and absolutely necessary to progress at chess.
- You can over focus on what may not even be the best move your opponent could make. We are all guilty of 'dream chess' (not 'hope chess') where we see lines that could be ultimately bad for our opponent before we see the the moves that could be bad for us. If you get focused on one of these, then your opponent plays something completely different and dangerous for you, then it can be quite distracting and you've just wasted a bunch of energy on a pointless persuit.
- Unless their move is forced or a logical recapture, you can not be 100% without a doubt sure what their move is going to be. So trying to calculate lines they may not even play is a waste of energy. Even if there are only 3 possilbe moves, and they make 1, you have wasted energy calculating 2 lines that were never used.
It's not an easy habit to start. But once you get it going, you will see a new dimension to your games.
I do feel like overthinking it without much of a reward. Maybe now I can play chess a little more relaxed and thats great... ill try!
thx
I need some advice on what should i be doing/thinking when my opponent is calculating his moves .... anyone has any ideas?