Diaphragmatic breathing is what natural/normal breathing is, and for example babies automatically breathe with their diaphragm.
Diaphragmatic breathing helps:
Decrease the work of breathing by slowing your breathing rate.
Decrease oxygen demand.
Help the lungs fill more efficiently
Use less effort and energy to breathe.
Bonus: it also helps tone your abs :>
PRACTICE:
Lie on your back on a flat surface with your knees bent. You can use a pillow under your head and your knees for support, if that’s more comfortable. Avoid doing it in bed to prevent the likelihood of falling asleep.
Place one hand on your upper chest and the other just below your rib cage. This will allow you to feel your diaphragm move as you breathe.
Breathe in slowly through your nose so that your stomach moves out, causing your hand to rise. The hand on your chest should remain as still as possible.
Tighten your stomach muscles, so that your stomach moves in, causing your hand to lower as you exhale through pursed lips. The hand on your upper chest should remain as still as possible.
When you get comfortable with the practice, you can perform it while seated, which allows you to do it anywhere.
Sit comfortably, with your knees bent and your shoulders, head and neck relaxed.
Place one hand on your upper chest and the other just below your rib cage. This will allow you to feel your diaphragm move as you breathe.
Breathe in slowly through your nose so that your stomach moves out against your hand. The hand on your chest should remain as still as possible.
Tighten your stomach muscles, so that your stomach moves back in, as you exhale through pursed lips. The hand on your upper chest must remain as still as possible.