When you repeatedly move in a specific way, your brain becomes very efficient at signaling that motor pattern. It’s as though repetition creates grooves in your brain. This happens because of a special fat called myelin. Myelin is like insulation on a wire. If you repeat the same motor pattern over and over, myelin accumulates around the axon of neurons in a shape reminiscent of a sausage ; the sausages increase the speed at which impulses travel along the nerve fiber.
Myelination of motor patterns is how you can ride a bike without consciously peddling the wheels. But a bike is difficult to ride incorrectly, because the skill is so simple, and because the bike forces you to move in a certain way.
In more chaotic and skill-intensive environments like, say, CrossFit, if you repeat the wrong pattern over and over, then you are wearing the wrong grooves into your brain. Re-grooving the correct pattern is the source of many frustrations. All of those tiny sausages in your brain stubbornly defy your efforts to do stuff the right way, because you’ve created the wrong neural pathway.
This is the biggest problem with doing CrossFit in your garage, or with your friends,or in some other wise where a coach isn’t present. It may be fun to fling some weights around, but if you’re grooving the wrong pattern, nobody is there to set you right.