I sugest you to play it rotating your wrist instead of doind lateral movements, try to make a circle, well, you may not be able to do a perfect circle with your wrist while playing.. an oval is more the shape you will get while moving your wrist..
then.. you will notice your hand more free with less tension, tension is only used when you drop your fingers to the keys, then you have to relax.. this takes time to practice and to perfect.. the only way to get rid of tension is practicing slowly with this rotation of the wrist, this way your brain captures the movement and after 300 repeats (or less depending on your hability) the movement is acquired and it will come out automatically