If 'Smartmove' is turned on, when you click on a piece that has only one legal move, it will make that move instantly. Also, when you click on a square that only one piece can move to, that move will be made instantly.
It can save you some time, but it can also cause you to make an unintended move. Lets say you miss you are in check and you click on your queen. The queen is placed between your king and the attacking piece.... BYE BYE queen.
Menu above: Edit > Settings > Playing 2 Tab Under 'Move Assistant' you can check/uncheck Smartmove.