How do I use the analysis engine?

When you are on a game board,  the icon to the far right (looks like two gears) will initiate the Stockfish analysis engine.  You can toggle it off by pressing the button a second time.

 

If you are observing a game, you can initiate analysis by clicking the hammer icon (second from left) and selecting  'Start Analysis'.   You can toggle it off by going back to the same menu and selecting 'Stop Analysis'.