I would imagine it’s the same with all swim coaches. I’m always being asked by parents and swimmers why swimmers need incredibly strong cores in order to swim fast. Much stronger core strength than land-based sports. And it’s true and obvious from watching the sport. Swimmers need to have incredibly strong cores.
It took a while for me to come up with an analogy to explain this to new swim parents and inquisitive swimmers. In order to run fast, runners use their bodies to apply force to the ground through their feet. Anybody who has run on shifting sand can immediately tell you that it takes more effort and you run more slowly if you don’t have that stable ground to work with.
Ice skating is a little more complex, but any ice skater can tell you something similar about ice. As long as your skate doesn’t slip, you can apply maximum force through to your feet.
In other words, the most effective way to move forward is to use your body to apply force to a stable platform.
But swimmers don’t have an external and stable platform. We swim through the water, which is constantly moving. So in order to effectively use our strength to move forward, our core has to be our stable platform. When we grab a lot of water and pull/push that water behind us, the core of our body has to be able to handle those forces without losing its stiffness. And the more stiff and stable our core is, the more efficient our propulsive efforts are.
So the next time a parent asks about core, or the next time a swimmer complains about doing more core training, you can just tell them. Runners have the ground. Skaters have the ice. Swimmers have their core.
Coach Rick - Mighty Tritons Swimming 
The buoyancy of the air inside rip cage and lungs is a huge force, when our body deeps into the water. So we must stablize this air-tank using muscles which involves in breathing.
Of course, I agree with you on the importance of core muscles.
It’s easier to push a dry piece of spaghetti (analogous to a strong core) through the water than a wet one (weak core)!
Swimmers need to know where their core strength originates. I ask them to pair up (on land) and push their (relaxed) partner to the side. The partner then resists and determines where strength is coming from. Finally get the right answer – the abdomen. Water drill – start a sprint away from the wall in a front lay-out instead of a push-off. They learn to engage core body strength.
Having very recently returned to the sport as an adult (stopped at 17, re-started at 31), I’m finding I have a heightened desire for awareness of how I’m moving through the water – either that or I just don’t remember thinking quite so deeply about it all those years back!
Of course, I always knew that maintaining a streamlined position in the water was one key to swimming faster, but the visualisation of the core being the tool by which we keep our torso stiff is a great new perspective. I’ll be making a conscious effort to feel this in future. Thanks.
Another perspective I’ve found interesting (but which may seem obvious to others), is that rather than trying to “move the water backwards”, isn’t it better to consider one’s self pulling their body through the water (thinking of the arms of course) with the water not moving at all? A sort of visualisation I’ve been toying with is the idea of traversing a set of monkey bars, or a ladder lying flat horizontally, from above, using the hands to grasp each rung and pull the body along.
I think perhaps my (fairly simplistic) visualisation misses a key point made in this article, in that, in order to make each pull of the water as effective as possible at producing forward motion through it, we definitely need to keep that core rigid.
Have just come across your site today and finding it very interesting. I’ll watch with interest for future articles and try to post some comments that disagree with your view!
Thanks Chris, I love the process of trying to find new ways to explain, educate or illustrate various concepts. No one idea has it all, and not all ideas work with everyone. I had a mentor coach who used to regularly ask me what new ideas and visualizations had I come up with. I also needed to have one ready for him.
The one where the hand stays still and the body moves over it is a great one, but I’ve also found that too much emphasis on that creates a great early catch but often swimmers abandon the tail end of the pull. So for me, its a constant process of educating swimmers. By the way, you may like The Importance of Simple Concepts https://coachrickswimming.com/2015/05/08/the-importance-of-simple-concepts/. It’s by far our most powerful tool in creating a natural swimming motion for all of our swimmers.