Rest is an essential part of growth and movement. The most restful person may not be the one who sleeps twelve hours a day, but the one able to grab catnaps while cruising at 500 miles-per-hour at 35,000 ft. on an airplane. Learning to act when it is time to act, and rest when it is time to rest, is the key to obtaining the peace of mind which keeps one's attention focused on the job at hand.
The image is of the spine, which serves as a switchboard for all the nerves of the body which mediate movement. When the spine is kept flexible and healthy through proper rest and relaxation, even active movement can be undertaken without strain.
