You can set up any position, either by using the Edit mode radio button or by pasting a position in standard FEN notation (explained here).
To analyze a position, select the uppermost radio button Inf for 'infinite' thinking.
Thinking then begins and ends by clicking on the Think / Stop thinking button.

In normal play, you do not use the Think button because, after you make a move, the computer starts thinking immediately.
If you do not want this, select the Only user radio button, which enables you to make all moves, for both White and Black.

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