“Permanent Brain” is the term used to denote the program thinking on the opponent's time.

 

After it has executed a move, the program tries to guess what the opponent will play, and starts calculating a reply to that. If it has guessed right, it will play its reply quickly - sometimes immediately. The program shows the move it is expecting in the engine window.

 

 

In engine tournaments, if two engines are playing against each other, using permanent brain slows down the side that is computing the next move (because it has to share resources with the other side's permanent brain).