Belly Bottom is a technique that is used in extremely shallow conditions, and as the name indicates, it involves lying in shallow water where your belly actually rests on the bottom, as seen below. When using this technique fins are completely unnecessary and not recommended. To move, simply raise your body using your arms and legs (like a pushup), crawl, and then rest on the bottom at your new location.
Safety Tip: Your soft underbelly will be in contact with the substrate. Be very careful in around sharp rocks, glass, fire coral or urchins! Lower yourself slowly and be prepared to get up if something starts hurting.