Consciously accessing information from the subconscious mind is the purpose of visualization.  In visualization, the conscious mind establishes a link with the subconscious mind to help you create what you want in your life.  Therefore, understanding the way the brain works is the first step in managing your mental state. 

Here are the basics.  The brain has billions of cells, most called neurons or nerve cells, that process and transmit information throughout the body's nervous system.  Brain activity can be tracked and measured by an Electroencephalograph or EEG that records the electrical activity produced by the firing of the brain's neurons.  Based on EEG frequency, there are four main types of measurable brainwaves:  Beta, Alpha, Delta, and Theta.

Your brain operates in the beta state when you are fully awake and involved in day-to-day activities.   The beta frequency range is from 12 Hz to about 30 Hz.  Low-amplitude beta is often associated with active, busy thinking and focused concentration. You operate in low-amplitude beta when your eyes are open, and you are fully alert.  Low-amplitude beta is actually a high mental state, and you allow internal and external suggestions to register only if they "make sense" to your mind.

Beta amplitude increases as you get more excited.  Getting charged up about a sport that you are watching puts you in high-amplitude beta.

Alpha brainwaves are slower with a frequency range from 8 Hz to 12 Hz.  Closing your eyes and beginning to relax automatically produces an alpha state as will slow, deep breathing.  The alpha state is the space in between waking and sleeping, and, at the lower cycles, occurs when you are in a light sleep.

At the alpha level, you are very open to internal and external suggestions once the mental mind has been quieted. Though your eyes may be open, daydreams can also produce a deep alpha state as can connecting fully with nature.  When you are in alpha, it is easier to access the subconscious mind and direct it to help manifest what you want.  Are you beginning to suspect where visualization comes in?  Deep relaxation?  Eyes closed?  Suggestions to your subconscious mind?

Scientists have found that listening to information while in a deeply-relaxed alpha state actually accelerates learning. This is also true for those in the theta state.

Theta is the frequency range from 4 Hz to 7 Hz.  When you are in the theta state, you are usually sound asleep.  However, those who have meditated for awhile or actively use imagination and deep relaxation can access the theta state while still remaining awake.  The theta state is associated with creativity, and those who can stay awake while in a theta brainwave state can uncover their most creative and unique ideas.

Delta brainwaves have a frequency up to 3 Hz, and delta is the slowest wave.  Delta is the brain activity of deep sleep, of deep healing and regeneration.  This is the realm of the unconscious.
Lower frequency ranges, then, are often associated with sleep, dreaming, super learning, accelerated healing, and a release of growth hormones.  These lower frequency ranges may be accessed through relaxation, meditation, active imagination, visualization, and physical practices like yoga and tai chi.  (There you have it!  Visualization is actually good for you.)

High-frequency ranges bring mental focus, reasoning, thinking, logic and critical thought processes, all to make it possible for you to interact with the external world.  Simply keeping your eyes open will maintain your beta state.

If you knew all of this, you're smarter than a fifth-grader.  However, knowing something and acting on it are two different things.  All brainwave states are important and serve important functions.  Any fifth-grader knows that.  The trick is in learning how to manage and maximize your brainwave states.  That becomes your job as an adult.