You may have heard how a note on a violin, if sounded repeatedly in rhythm, will start in motion vibrations which will in time destroy a bridge.
The same result is true when a regiment of soldiers crosses a bridge, the order being always given to “break step” on such an occasion, so the vibration will not bring down the bridge and the regiment.
These examples of the effect of rhythmic motion should give you some idea of the effect on the body of rhythmic breathing.
The whole system catches the vibration and becomes in harmony with the will, which causes the rhythmic motion of the lungs, and while in such complete harmony will respond readily to orders from the will.
With the body so attuned, the Yogi finds no difficulty in increasing the circulation in any part of the body by an order from the will. And in the same way he can direct an increased current of nerve force into any part or organ, strengthening and stimulating it.