I was taught, and I’m sure many of you were as well, to perform an S shape pull underwater. The S shape motion has the swimmers hand entering thumb first, followed by the hand shifting out, back in, and then back out again forming what appears to be an S underwater.
Since my age group swimming days, we have learned that this is not the most efficient, economical, or safest pull motion.
Upon inspection, one thing we know for sure is that if a swimmer leads with a thumb first entry (i.e. palm facing the side of the pool) her shoulder is internally rotated. If the swimmer presses the water laterally to perform the S motion with the thumb down a good deal of stress is placed on the shoulder, which could lead to injury.
Aim to have all fingertips enter the water at the same time.
A good paddle to train the hand to properly enter the water can be found here, My Favorite Paddles
It is my opinion that the biggest drawback to teaching the S shape pull is that swimmers often exaggerate the motion attempting to create a big S. Pressing the water out to the side does not produce propulsion, it just moves you laterally. Pressing the water back is what moves you forward.
Although it may appear that some elite swimmers have this motion in their stroke I would argue that their intention is to move forward as fast as possible.
Bottom line: Focus on a clean entry, with your fingertips entering the water at the same time and press the water back.