The physical aspect of the body is just one fifth of the body of a being. There are four more dimensions to it, which one becomes aware of and visible with the grace of the Guru, through the practice of yog. There is a subtle movement of prana through these five sheaths of existences (koshas). It's through this prana that these layers communicate with each other, and with divinity and the life processes go on. What a yogi looks at is not just the physical body but these sheaths, which are also called the human aura. Looking at the aura one can tell the state of mind of a being and also the problematic areas of the body.
The connecting factor and the communicator between these sheaths is the breath, the physical breath and also its subtler dimensions. It's through this process of breathing that one can achieve any state or level of existence / consciousness.
The various things, which one sees in Creation are all an aspect of that one prana, but differentiated from each other by the various frequencies at which they vibrate. For example a piece of stone vibrates at a different frequency than that of a piece of wood. Similarly all colours vibrate at different frequencies, hence they look different. The differentiating factor between the various parts of the physical body and also its etheric layers is also this different frequency of prana. Two people look and behave different because they vibrate at different pranic frequencies.
To an accomplished yogi all layers of the being are visible as different colours, including the physical layer.
The control over the physical and the etheric breath, which is brought about by Sanatan Kriya, helps one to access the higher frequencies of Creation. It's through the movement of breaths that one can awaken the higher senses and also access those frequencies which have the ability to bring about a change in one self; the change may be physical or mental.
Breath is the strongest communicator of prana and also its frequency and duration determines the level of evolution and also the state of the physical health of a being. All frequencies of creation are existing around us; the way we breathe determines the level that we can connect with and also the diseases which we will develop.