The underlying principle to all of yoga is that with the release of obstructions in the subtle body (energy body), there is greater physical, mental and spiritual well being. Yoga is not about trying to get something that we don't have, rather it is about uncovering and releasing the obstructions that we do have so that the body's innate intelligence can then reorganize itself in an optimal way. Until we begin the practice of yoga, we may not be aware that these obstructions exist because they operate beneath the level of our consciousness. Through the practice of yoga, these long stored energy patterns rise to the surface of our consciousness facilitating the process of letting go.
A useful analogy is a farmer who has to break a dam in order for the water to flow to his fields. The dam is the obstruction and the water flowing to the fields is the inflow of profound intelligence which like water seeks its own level and will do its own thing. The implication is that there is a certain natural state of being in the system which can arise when everything that's in its way is taken care of.
In the same way the practice of hatha yoga may feel like a lot of exertion at first. However with regular practice and under the tutelage of an experienced yoga teacher, the body's natural intelligence transforms and heals. With sustained practice the body reorganizes itself optimally allowing prana, the universal life energy, to flow freely just like water finding the path of least resistance and nourishing everything along its way.