Freestyle 'skating' drill

This, to me is one of the most important drills for freestyle.  Decent rotation is important for a number of reasons.  It helps with breathing.  Whether you are breathing on one side or two.  But if you are breathing both sides, it will help you rotate far enough. Majority of swimmers don't rotate enough on their 'weak' side.  Doing this drill where you are 'skating' 50% of the time on each will help even things out.

I call it the skating drill because you are sort of skating on that front edge of your hip bone, that kind of sharp, pointy bit. 

Extends Reach and Leverage

Rotating properly also helps to extend one's reach.  This makes you more efficient.  You will find yourself going faster or minimally the same with fewer strokes.   And much less effort.  You also are going to engage more muscles. Your lats and core are going to do more of the work. As opposed to just arms and shoulders.

Shoulder Flexibility

The amount that one needs or wants to rotate will be different for each of us.  Shoulder flexibility enters in to this as well.  If there is little flexibility, one might feel they have to rotate too much.  There are a number of good stretches that one can do to loosen up your shoulders.  One of the best is straight arms holding the ends of a towel. Then drop it behind you from over your head with arms straight.  As the shoulders get looser one can shorten the grip on the towel.

Why I Use Fins

I use fins in this drill to strengthen and increase flexibility.  I do a two beat kick.  I do a hard kick on same side my hand is exiting.  Good for extra propulsion.  The larger/longer fins stretch my ankles. They keep them flexible and also strengthen my calves.  This type of kick helps me keep my stroke count per length at 15-16.  At a pretty good clip.  

Find your sweet spot.  When doing this drill, find the angle that is best for you. And make sure you do it on both sides to keep your stroke even.