This exercise serves many functions. By maximizing the inhale (and sipping in a bit extra, past your perceived capacity), you are both strengthening and stretching your inhale muscles, facilitating stronger and fuller inhalations. By maximizing the exhale (and pushing out a bit extra, past your perceived capacity), you are strengthening your exhale muscles, making you able to exhale more completely, allowing more oxygen in on the subsequent inhale, and increasing the endurance of these muscles. This is a critical factor for athletes. Remember, when your breathing muscles fatigue, you fatigue.