Is there some advantage using both dry suit and wing for buoyancy at the same time? You could argue that you always use both but I mean adding air to dry suit for buoyancy. Apart from more air = more warmth. ?
Ok, thanks. I use wing for buoyancy as it is easier for me. I am over weighted with steel cylinder without any lead. Is there any theoretical/practical reasons why to use both for buoyancy? Things like if you rupture your wing bladder you have some buoyancy left in suit...
Advanced techniques...
When I'm dropping down the shot line (that's the rope with grapnel anchor thrown in on to the wreck with a large buoy to mark the dive site). At the surface I'll dump the contents of the BCD and fall down the shot line, possibly hand-over-hand in the current, pausing periodically to inject some air into the drysuit to relive the squeeze. This can get quite severe if you're slow! The main thing is to get down as quick as possible. As I start to arrive at the target depth, I'll blow in some gas into the BCD to start slowing down the descent. Also the drysuit, but not too much.
When arriving on the bottom, I'll settle everything down, blow some gas into the drysuit to loosen it and "shake it up", often the legs need pulling up and the body moved around. Then I'll use my BCD (called a wing) for coarse buoyancy control. The wing effectively offsets the additional weight I need to dive with which can be lost or consumed during the dive (i.e. gas from my cylinders, my heavy reel and SMB, etc.) which will need a good 3kg/7lbs of gas in the wing (3 litres multiplied by the depth in atmospheres).
I can then use the drysuit for fine control of buoyancy. With a rebreather, which I dive, you can't control your buoyancy with your lungs as you can when you're using open circuit (blowing bubbles). The drysuit inflation is fed from a separate cylinder of air, so is cheap compared with using the expensive helium mix used in deeper dives (which the wing is fed from).
Also the drysuit is far easier to dump gas from: simply raise your left shoulder slightly. The wing needs you to move your left arm (which holds the torch) around to your hip to dump from. This is important as it's a lot easier to adjust with your shoulder dump than your hip dump.
All of the above is during the bottom phase of the dive.
When ascending, you need to take a considerable time in doing your decompression stops which can easily be a long time, certainly well over an hour. On these stops, just as anyone doing a 3mins at 5m/18' safety stop knows, time passes slowly as you're not moving. This means you get cold, so you need will add more air to your drysuit to help keep you warmer than being shrink-wrapped.
This is what I mean by being far more nuanced than an Open Water instructor dogmatically explaining how you must do it. However, when you start, it's a lot easier to simply get rid of the squeeze and use the BCD and your lungs for buoyancy control. The rest you'll work out over a few dozen dives.