The more detailed and real you can make your visualization, the more effective it is likely to be.
Do you ever feel that your arms and legs prevent you from swimming better?
After all, lack of knowledge clearly is not the factor that's holding you back. You're already familiar with the various elements of proper freestyle technique: floating high in the water, rotating from the hips, pulling with a "big paddle" and the rest. You have a very clear mental vision of what your body should do.
During workouts, your brain commands your arms and legs to move just like Michael Phelps's limbs do when he swims freestyle, but the muscles do not (or cannot) obey. If only you could get rid of your body and swim in your mind with the perfect technique you picture and intend there.
Actually, to some degree, your limbs really do get in the way of improving your swimming, and you can in fact refine your freestyle technique by practicing without your arms and legs. What makes this possible is the fact that the motor programs stored in your brain and activated to control your freestyle stroke are far more malleable than your muscles, which execute these programs.
In other words, your brain's motor centers can imagine and intend alternative ways of swimming far more easily than your muscles can adjust their movements. Thus, by temporarily replacing your real muscles with imaginary ones—that is, by visualizing yourself swimming—you can practice alternative techniques with greater freedom and make it easier to get your muscles to do what you want them to do when you return to the pool.
Your freestyle swim stroke—like every other motor skill—is produced through two-way communication between your brain and your muscles. The motor centers of your brain store programs for your freestyle stroke that were developed through previous practice.
When you decide to begin swimming, your brain selects the appropriate programs and executes them by sending electrical signals to the muscles, causing them to move in the programmed pattern. As you swim, your muscles send a constant stream of sensory feedback to your brain, providing data that enables your brain to refine and adjust the stroke.
It's this sensory feedback, or the feel of your muscle movements, that constrains your ability to fiddle with your stroke in ways that make it more efficient and powerful. When you practice your swimming through visualization, you replace real sensory feedback from the muscles with images of correct technique that you have captured by studying photographs and instructional videos and by watching better swimmers at the pool.
Armed with this data, you can easily see and feel yourself swimming with better technique while lying in bed with your eyes closed. When you imagine yourself moving, you activate the very same neurons (brain cells) that become active when you actually move. Frequent mental practice causes these patterns of neural activity to consolidate into newer, better motor programs for swimming.
When you return to the pool you can draw on these new programs. With your muscles factored back into the equation, you won't find it quite as easy to swim like Michael Phelps as you did in your bed, but it will be decidedly easier than it would have been if you had not used visualization.
Most athletes are unaware of just how much control the brain has over athletic movement. Your brain is the puppet master; your muscles are mere puppets. In fact, they are totally replaceable. By implanting electrodes inside the brain's motor centers, medical engineers have enabled quadriplegics to play video games with their thoughts.