I used this to increase cardiovascular and lung efficiency and volume. Start at 3-1-3, then progressively train to go to 3-2-3, 4-1-4, then 5-2-5, etc, inserting shortened (say 3-1-4) but deeper inhales or skipping the hold when getting short on breath. Inhaling has a natural tendency to slightly increase your heart rate while exhaling does the opposite, so the trick helps a lot in getting in control of a racing heart.