Premove is an option that will allow you to play your next move while your opponent is still thinking on his time.  It can be turned on and off by going to the Playing 2 tab in the Settings Dialog.

Once the option has been turned on, you can premove your move during your opponent's move by picking up your piece, dragging it to where you want it to be moved, and then dropping it. When your opponent has finished making his move, the Dasher interface will autoplay your selected move (the move that you just dragged and dropped) with no loss of time except the tenth of a second required by the server on all moves to prevent abuse.

premoves are highlighted in green and played moves are highlighted in red:

premove castle:
 played castle:

This saves a lot of time but has a drawback. Your move is made no matter what your opponent plays, unless it is not a legal move anymore.

In the above example, no matter what white plays, black will castle O-O on his next move.

Use premove with care. If white were to play a3, for example, attacking the black bishop on b4, black will still premove O-O and lose the bishop! This is a calculated risk in exchange for the few tenths of a second that you may save on your move, which usually means that it is best used in fast game types such as Bullet and 1-minute.

You can cancel a premove by left-clicking on any piece or empty square on the board.

See Also:
Move Assistant
Minimum move time
premove -- command line help: