The Buddha talks about how the wheel of life keeps on moving, and how the wheel turns around a fixed center. If you only look at the periphery of the wheel, you are looking at the outer form. If you become capable of accessing the hub, the core center, you will be able to connect with something essential, timeless and eternal. When we focus on form, and only look to the periphery, we find ourselves repeating the same superficial motions again and again and again.
Buddha calls life a wheel because things go on habitually repeating themselves. The more you mechanically repeat the same form of the poses, the sooner your practice will go flat as a boredom and dullness sets in. If you’ve been on the mat for enough hours in your life, you’ve probably hit that kind of rut in your practice at one time or another, likely without realizing why. A singular focus on form robs you of freshness and creativity, and the practice quickly grows stale. Buddha calls this unconscious repetitious turning of the wheel, samsara. Outside of this repetitious rut is where you will discover nirvana.
Keep expanding in your yoga practice and your life—you need not to get caught up in the periphery pursuits. Instead, reach deeper and get to the source of your experience.