I am a reasonably good swimmer (do 2km in 45 mins) but recently went to lessons to get faster and now am slower whilst I get to grip with my new technique. This does involve hip rotation (I never did this before) and its a great core workout. I was told to google Michael Phelps slow motion videos as these are great examples of technique - you can see his hips almost break water on the rotate! Its all about getting a minimal line in the water.