A back-inflate BCD is similar to a BCD jacket, but has its air bladders located on the back rather than around a divers torso. For divers who feel squished when inflating jacket style BCDs, this is a welcome relief.
By relocating the air bladder, the chest area is decluttered, resulting in a more minimalist setup. This is especially popular amongst freedivers and spearfishing enthusiasts who are accustomed to diving without the burden of so much gear.
However, the main advantage of this type of BCD is the diving position it facilitates underwater. By relocating the air bladders, divers are more stable, floating in a horizontal position, which is often referred to as trim.
By improving trim, you become more comfortable and will improve your aquadynamics. While this comfort comes from a lower centre of gravity, your new streamlined diving position will require less energy and oxygen while diving, ultimately allowing you to dive for longer.
The only downside of such a setup is the floating position above water. While an inflated jacket-style BCDs keep you in a stable horizontal position on the surface, back-inflate versions tend to keep pushing you into a horizontal position, often submerging your face.
This issue can, however, be avoided by slightly deflating your BCD, or shifting your center of gravity backward. With a bit of practice, this is usually not a major problem, but is something to consider. If you are doing many shore dives requiring long swim outs, maybe this isn’t the best option for you.