Seakdiver:
A backplate is a piece of steel or aluminum that fits agaisnt the wearer's back. There are plastic plates, but I've never heard anything good about them.
The backplate attaches to the wearer by means of webbing, most often a continuous piece that loops over the shoulders and around the waist. It is fastened at the waist using a buckle.
A Wing is attached to the backplate. A single tank can be attached using tank bands and, in some cases, an adapter called a single tank adapter, or STA. Doubles are bolted directly to the plate and banded together.
Regardless the wing fits between the tank and the plate.
In addition to the webbing, you wear a crotch strap. This is a piece of soft webbing that runs from back to front between the legs. It helps keep the rig in position.
You can see excellent pictures of backplates and wings at this site.
http://www.halcyon.net/mc/harness.shtml
Page down for a good look at the plate and webbing.
In my opinion, there are several advantages to a backplate when compared to a BC. Most of these are not gender specific.
1. The BP is very streamlined. The diver's chest and stomach are are kept free of drag creating pockets and padding. This means that it takes less effort to move throught the water, particularly since drag increases exponentially in relation to cross section.
2. The BP places weight on the wearer's back. This is directly over the largest source of lift, the lungs. This makes it easier to maintain a horizontal trim.
3. Having weight on your back reduces the weight worn on the waist. Enough said.
4. When properly fitted, BP's hold the tank quite securely. My experience with BC's is that they allow more movement than BP's, which creates trim issues. It's also annoying.
In addition, in my experience BP's hold the tank closer to the body and a BC. This allows you easier access to your valves in the event of an emergency that requires shutdown or when reacting to a roll off.
5. There are fewer failure points on a BP than on a BC. Fewer things to fail means a more robust rig. While this may be a nominal increase over BP's, that's not much consolation when someone drops a tank on your BC's Quick Disconnect, leaving you without a BC until it gets fixed.
6. I can think of two potential advantages that are more specific to women. However, I claim no personal experience other than fitting female divers for BP's.
First, some female divers have told me that they have experienced difficulty finding a BC that fits. Because BP's rely upon a continuous piece of webbing held in place by keepers and a buckle to keep them in place, they are almost infinately adjustable.
Second, there are no chest straps to interfere or create discomfort. Thus, for example, large busted women don't need to worry about finding a BC that fits both bust and waist.
Other manufacturers, including OMS and Zeagle, make BP's. FredT, a member of the Board, makes a very nice product as well.
