Yoga can not be bound by simple words, nor explained through lengthy speeches. The term refers to both a state and a practice and means many different things to different people. Most of the contemporary world identifies yoga only with physical poses but the literal and scholarly definitions point towards union. Nowadays, we often hear that union will occur between the body, mind and other aspects of the self. Yet classically, these aspects were never understood to be distinct. They were seen as one. So how can they become unified if they were never separate to begin with? No cell of our being exists in isolation; just as no human exists outside of the universal reality. The union that yoga offers is a reunion, the process of remembering and tangibly realizing the interconnectedness and relationship between all things.
So what does that have to do with physical flexibility and posing? Without doubt, yoga asanas bear fruit on a obvious level. Regular practice offers a variety of benefits including increased flexibility, strength, circulation and general well-being. We also experience more subtle benefits such as heightened concentration, gratitude and patience. Yet all these benefits are to be merely accepted and not actively pursued. Simply noticing the transformations within our bodies is a portal to observe ever more subtle transformations within other fields. Asana practice builds an inner stillness that may enable us to realize our own truest nature. Patanjali, the sage credited with defining yoga, has put forth "yogaś citta-vṛtti-nirodhaḥ", yoga is the cessation of fluctuations in consciousness. With regular practice, we may become aware of a lessening of the turnings within our mind. This encourages us that our yoga is indeed effective, that continued practice may bring even sweeter results. I believe that I agree with most practitioners that the fruits of yoga can not be sufficiently explained, they must be experienced.