Side mounting is mounting one cylinder on each side of your body. A BC harness system is responsible for holding the cylinders in place. Side mount was originally developed to allow the exploration of restrictive underwater cave passage that was not accessible by standard back mounted cylinders.
Side mount configurations allowed the diver to maneuver through much smaller passage by placing the cylinders on the divers side, thus reducing the overall girth of the diver. Side mount rigs also allow for the easy removal and replacement of cylinders underwater, enabling the hard-core cave explorer to squeeze through even smaller restrictions by removing one or both of the cylinders and pushing the cylinder in front of them through the restriction.
Side mount cave exploration is not a new technique by any means. It has been around for many years, but only since 1999 has it gained any type of recognition as a viable method of exploration. Some of the first side mount cave explorers where Woody Jasper, Lamar Hires, Brain Kakuk, and Wes Skiles. They pioneered their own side mount rigs in their garages and made them work. Continued modifications to these original rigs brought side mounting into the more mainstream.