We most often think of water as a whole body or continuous substance. But really, from another viewpoint in physics, it is a soup of individual water molecules, or little electrical balls all jumbled, bumping and swirling together. And if you want to swim forward in the water, all those little balls are filling the space ahead that your body needs to occupy.
In terms of hydrodynamics, if you want to swim forward, you needs to get the water molecules in front out of the way so your body can occupy that space instead. If you want to move forward faster, you need to get those water molecules out of the way more quickly. In order to get water molecules out of the way more quickly, you need to get those molecules moving aside uniformly. The more orderly, the more smoothly they are made to flow, the more quickly they go aside.
Now think of those water molecules in front of you as a crowd of people exiting a packed theater. If the people are orderly and calm moving toward the clearly marked exits in their section, all the people can clear out rather quickly. If they are panicked and rushed, not sure where they need to go, then there is a traffic jam, trampling and clogging at the exits, or worse. This disorderly movement inhibits the exit of people.
Likewise, in the water, shoving water molecules haphazardly, with abrupt and hurried action creates more turbulence, which inhibits the movement of water molecules out of your way. When you shove molecules like that, like panicked people shoving each other around, they increase resistance to moving out of your way. At any speed you intend to swim the better you use carefully crafted movement patterns to clear the molecules smoothly, uniformly, even calmly out of your way, the more quickly your body can move forward and occupy that space ahead. The quicker you get those molecules to move out of the way determines how effective you are. The amount of energy you use to do this determines how efficient you are. You can be powerful in your actions, but it must be orderly and smooth in order for it to be effective and efficient.
You can more easily see this wisdom applied in other aquatic sports…
Now imagine a fire alarm going off in a crowded theater. You are an emergency director standing in the middle of the room giving exit instructions to that crowd. You need them to get out as quickly as possible, but to do this you have to apply great attention and effort to guiding their movements. You will give clear, precise instructions in a firm and assuring voice, keeping everyone orderly and calm, moving in the direction the people in each section need to go. Or you can let go of attention to such details and just shout and yell and try to force people to move all at once, letting them try to figure out individually how to push and shove each other out the door or out of their way.
Which way is going to clear that theater faster? Getting the crowd to empty from the room in the quickest way requires them to be calm, orderly, patient as they follow instructions. This is totally counter-intuitive to human instinct in a mobilized state. This is why we need fire drill training, and emergency directors, signs and lights to keep people calm and orderly. It’s not natural, but it is something we can train a group of people to do under stressful conditions.
Following the analogy, since you are the director of your own body parts influencing the crowd of water molecules ahead of you, which way do you tend to get them out of your way while trying to swimming faster? Do you instruct your body to move those molecules in an orderly and calm way? Or do you attempt to push and shove the water out of the way, throwing concern for order overboard?
There is a reason the fastest swimmers in the world, especially in distance events, tend to look relatively smooth and calm compared to the rest of the field. Counter-intuitively, swimming in such way gets water molecules out of the way the quickest, with the least energy expense. You can have them both… if you train for both together. Certainly, all of the fastest swimmers in the world are effective, but not all are efficient in how they go about it, though the few effective-but-inefficient ones near the top seem to get away with it for a while. If you could be faster with either approach, which kind would you like to emulate for the rest of your swimming life?
Now consider how many ways you see people being taught to swim faster. Some are even encouraged to whack, hack, chop, splash, slap, yank, kick, punch at the water (maybe even yell at it too). They can do that all they want, but it will cost a lot more energy, and most of those actions won’t get them moving forward much faster, and those certainly won’t make them last longer.