One of the big risks for a technical diver is decompression sickness. It occurs when nitrogen gas cannot exit the body thru respiration on ascent as the ambient pressure decreases. Therefore stops have to be executed on ascent in order to avoid a pressure difference too big so nitrogen bubbles can be formed. Buoyancy control is very important to control the stops on ascent and increase efficiency for off-gassing of nitrogen. Another important skill is proper breathing which makes metabolism efficient. Trim, which we call a proper tech diving position such as horizontal with a slight arch in the back, looking forward and knees at a 90 degree angle with fins pointing back. Trim prevents stirring up sediments on the bottom as well as most efficient movement in the water. And of course proper finning techniques to avoid overexertion are mandatory to make a diver achieve maximum performance. Now you might say, we want to have fun when we go diving and not be a machine.
Because of the difficulties with single orifice doubles and backmounted independents, doubles were introduced to backmounted diving. Independents are two cylinders that are strapped to one's chest with separate regulators. A 1st stage failure would result in only one cylinder being used by the diver. A single orifice double was two cylinders connected together by a valve and one regulator. All gas would be lost if the first stage of a single orifice twice set failed. Today, both problems are eliminated by the isolated manifold doubles. Each cylinder can be operated independently. However, the diver can still inhale gas from both cylinders using one regulator. The doubles are usually held together by metal bands. The valves are connected with an isolation manifold so they can be separated if necessary. Doubles diving with backmounted doubles is similar to a regular single-cylinder backmount. It offers a vertical profile that is the same.
