If you do a regular tack, the boat slows down during while you come about.  But if you do a roll tack, it maintains its speed.  However, if you are racing, and you come out of a roll tack faster than you go into it, you have broken a racing rule by using your body for propulsion.  Roll tacking is an example of using kinetics.

come about - turning when going up wind

tack - turning far enough so the sail goes from one side to the other