Why should I place my hand on my belly during a breathing exercise?

In some breathing sessions, we put our hands on our belly (or one hand on the belly and the other on the chest) to follow the movement of the breath and to make sure we are taking deep diaphragmatic breaths.

Diaphragmatic or belly breathing is quite a straightforward technique and is used in many sessions, so it is essential to get it right.

You can find more benefits of diaphragmatic breathing here. 

If you are new to belly breathing, lying down and placing your hands on your belly right below your rib cage is very helpful during practice. When you take a deep breath, the diaphragm, the dome-shaped muscle that sits below your ribs and above your stomach, contracts, flattens and pulls down. It creates a vacuum that draws air into the lungs. When you breathe out, the diaphragm loosens up, goes back to its original dome shape, forcing the air out of the lungs.

As you breathe in, the hand on your belly should comfortably rise and as you breathe out, it should fall. Putting one hand on the belly and the other on the chest helps you to see how much your belly and chest are moving. Depending on the session's aim, you may be asked to keep the hand on the chest still to make sure that all concentration is on the diaphragm.  

While taking diaphragmatic breaths, you may also try placing your hands on your sides below the ribs to feel the expansion that occurs sideways. Breathing is not only an up-down movement in your belly; the expansion is actually three-dimensional.

You should make sure that you are not forcing your belly to expand. It is possible to expand it using your abdominal muscles without moving air around at all. Of course, that is not the purpose. The expansion of the belly should result from air filling into the lungs.