What is smartmove and how do I turn it on? (BlitzIn)

If 'Smartmove' is turned on, when you click on a piece that has only one legal move, it will make that move 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 Tab   Under 'Move input' you can check/uncheck Smartmove.