The changes depend on a number of factors like the physical & mental condition of an individual.
The diet & determination of the person also matters as well as the nature of any illnesses, if present.
It cannot be explained by science how it exactly works but only the changes that occur in the body are evident.
Yoga takes action on the Chakras or centers of energy found in certain areas of the body.
Attention is focused on these centers allowing the energy to flow modifying the functions of the body to attain the appropriate balance.
There are physicians with open-minds who will not discourage the practice of mystic arts if it does well to the body.
But doctors who actually practice the exercise or have seen the benefits first hand would certainly recommend it as supplementary aid to the treatment.
The bad side to this is, when people expect too much from it only to find out that it does not produce the improvement that they seek.
Physical therapists would advocate this because it can improve posture and release tensions within the body.
There are several schools of yoga such as Yoga Sapta, Raja Yoga, Hatha Yoga, Tantra Yoga, Kundalini Yoga, etc.
They may differ in their underlying theories and there are dynamic practices as well as static practices.
There are exercises that focus on standing postures whereas others concentrate on breathing mechanics.
Hatha Yoga requires a lot of strength concentrated on the body while Raja Yoga develops the mind, which is a component of the Kundalini movement.
A practice popular in the West, Hatha Yoga requires a studio that is large enough for the participants to feel comfortable in and have no difficulty of movement.
Accessories essential to the postures are needed such as mats, horizontal bars, & ropes.
Music is important to help create an environment that is suitable for concentration.
There is no regulation clothing like in karate as long as your clothes are comfortable and allows ease of movement.
Shoes are not allowed because some positions of the foot are not possible when wearing something.
It develops the strength of the legs as well as the toes and the muscles of the foot are being utilized in the positions.
The purpose of all these postures is to get into a state conducive to meditation.
After a yoga session you should be capable of relaxation.