Here’s the short answer: The shortest distance between two points is a straight line. If you pull straight back underwater, your arm and hand will travel the least distance from front to back. If you can make your underwater pull distance longer, by adding an S-curve to it, you give yourself more opportunity to exert more backward force which can help you swim faster. A longer pull is a stronger pull.
But let’s be clear: we mean “longer” as in distance, not in time. Taking a long time to pull can be a slower way to swim, as Sheila explains fully in her books and introduces in these posts about gliding.
And of course there’s a lot more to the short answer! Here’s part of the longer answer in this passage from Sheila’s new book Swim Speed Strokes:
In the early 1970s, 20 years after Doc Counsilman’s first suspicions about lift and drag were raised, he presented his own research findings to the swim community. Using photos of the underwater stroke patterns of current swim stars—many from his Indiana University team, including Mark Spitz—Doc showed the curvilinear pull patterns (in all strokes) and offered an explanation as to why the best swimmers were stroking this way.
Doc first explained the limitations of Newton’s Third Law as it applied to swimming. He said that when a swimmer exerts a back-pushing force on the water, the water begins to move, and once the water is set in motion it offers less resistance to the swimmer. Doc explained the swimmer’s hand/arm would have to continually accelerate to keep up with the moving water. Accelerating in this manner, stroke after stroke, would be exhausting and an inefficient use of the swimmer’s energy.
Doc believed that Newton’s Third Law was effective for pushing water back in short increments but not longer increments such as the full length of the underwater stroke. He believed that by moving the hand/arm to an adjacent plane of water either to the side, below, or above the backward-directing path, the swimmer would find slower-moving or “still” water and could effectively apply Newton’s Third Law again.
This part of Doc’s explanation was easy for coaches to accept since it preserved most of the commonly held action-reaction beliefs of the time; however, the second part of Doc’s presentation caused a stir. He explained that the lateral and vertical stroking patterns were not just a necessary evil for the swimmer to maneuver their way to a plane of still water, but rather that propulsive power was actually generated by these movements—perhaps even more propulsive power than drag could offer. Doc then explained Bernoulli’s Principle and introduced the concept of lift to the swimming world for the first time in the sport’s history. His first published article on the topic was “The Application of Bernoulli’s Principle to Human Propulsion in Water” (1970).
Doc’s theory became known in swimming circles as the S-Pull pattern. He explained that just as a boat driven by a propeller is faster than a boat driven by a paddle, so too is a swimmer who navigates the hands/arms like a propeller (in an S-curve path) through the water faster than a swimmer who uses the hands/arms like a paddle pushing straight back.
As you might guess by continuously falling world records in swimming since 1970, there’s a lot more to this story. Doc’s research into swimming was complemented by Cecil Colwin and refined by Ernie Maglischo. In her new book Swim Speed Strokes, Sheila Taormina offers an overview of how Colwin and Maglischo changed how the sport things about swimming stroke physics–and what it means for swimmers today.
Swim Speed Secrets reveals the swimming technique used by the world’s fastest swimmers.
Swim Speed Workouts provides waterproof workout cards, drills, and training plan so swimmers can get in the pool and learn the fastest way to swim.
Swim Speed Strokes shows how to master elite technique in all four competitive swimming strokes.