By reducing the intake of oxygen while exercising, the body enters a hypoxic state and adapts, producing more red blood cells.
Oxygen deprivation causes hypoxia, this seriously shocks the entire body. The body then starts to breath more fully when there is oxygen. This can be practiced easily while just walking. Exhale and hold out the breath for a few steps. Keep this comfortable and start with 5 cycles every few days. Over time this can be built up and practiced with weights too.
This a popular training method to professional athletes in various disciplines and has been used as a therapy for treatment of nervous disorders, heart and lung problems and dealing with muscular weakness.
As a training tool this method needs clever application. Excessive practice can be very harmful. Its essential to incorporate this practice over time and a few days rest with only light training after a practice day. Later your body will have adapted a only require minimal rest.