In swimming, you ideally want to turn on only the muscles that are required to hold a position or move. And you want to fire those muscles only to the extent necessary. If you train the muscles to achieve the optimal body positions and motion using the least amount of energy for the desired pace, that's relaxation and economy of movement.
another way i like to express this is, what is the absolute minimal amount of energy you can expend in order to maintain your streamlined body position. more than this and you're wasting energy. less than this and your body will lose its streamline and flop around in the water.
CoachDavidShen
Imagine you have a loose slipper hanging on the end of your foot as you swim. 2BK should feel you are "flicking" your foot to discard it.It's a small, light and quick movement.
learn to lean into the stroke of the arm. this is a very subtle thing that coach shinji teaches. there are drills for this. this does cause the hips to remain high.
press the chest, or more accurately it is a point on your pec muscle just inside your arm/shoulder joint. it's not the armpit; we don't rotate all the way to the side any more.