Harnesses
Many white water buoyancy aids, as well as some sea kayak ones, feature a safety harness. These fit around the buoyancy aid below the arms and can be used for Live Bait Rescues (where a rescuer swims out into the river on a line to rescue a swimmer) or for anchoring a belayer onto the bank. They feature quick-release buckles to quickly remove the harness, and these are often (but not always) designed to release automatically past a certain load/pressure. Often harnesses have a metal ring at the back for attaching a rope with a screwgate carabiner. Use of a non-locking carabiner can allow the carabiner to accidentally clip onto other straps on the buoyancy aid during use, thus preventing the harness from being released, and as such they should not be used for attaching to a harness.
Many harnesses feature a metal friction plate which stops the belt from slipping under high-load situations, however these can also prevent the harness coming off when swimming and as such present more of a hazard than an aid. It is often recommended that the belt be removed from within the friction plate and only threaded back through if the situation requires an anchored belay.
Improvised harnesses (ropes around the waist, belts, or attaching ropes directly onto the buoyancy aid itself) can present a serious danger to the wearer, as they can not be released or may not be strong enough to withstand the load required.
Read more about this topic: Buoyancy Aid