it depends on how you rotate and swing and your hitting structure. the more important part is keeping your elbow at roughly the same distance from the torso if you have one of those relatively locked bent arm swings. If you have a swing where you lead with the elbow then usually from the elbow to the hand is kept loose and depending on your setup position at the end of takeback etc etc the elbow extension that occurs during the swing will do be pretty much 'pre-determined' and if you have your racquet weight configured to your swing then the lag in the head will pull back on a leading elbow fh which... nvm hard to explain. in any case there are many types of swings. you can lock you hitting structure with muscle contraction of course but nobody is 'taught' this. At most people are taught early on to 'maintain the bend' where they will consciously use contraction. Then there is the phenomenon where, as an example, players who play with an extremely loose grip, like i do, tighten the grip at contact automatically w/o thought. There's a reason that happens subconsciously but i forgot the name of the neural process that causes that happen (in regards to the grip tightening on it's own).