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.
Because side mount diving is unique and a very specialized form of diving, very few training organizations offer this kind of training. There is no official certification for side mounting. The NSSCDS does offer an introduction to side mount diving course that exposes you to alternate cylinder, harness, and BC configurations when you can’t use back-mounted cylinders.
There are side mount divers who can provide information and tips for side mount diving, gear configurations, trim, streamlining, and building a side mount rig. Some instructors who side mount can also provide help and training if you want to get into side mount diving.