If you can unfocus your eyes on demand, you are relaxing the ciliary muscles in your eyes. This results in the lens in your eye becoming thinner and flatter, resulting in a blurred image. The medical term for this is called negative accommodation and the eyes naturally do this to adjust the long distance vision. This is usually harmless, but it can lead to eye fatigue and strain and in rare cases, if done repeatedly it can disrupt your normal eye alignment. Please see your doctor if this happens.