If you start out five pounds heavy, which you do(more or less) because of the gas you're carrying, there needs to be enough air in your wing/BC to compensate for it, about five pints by volume. As you progress through the dive and become lighter due to the air you've used, you can release the extra air from your wing to compensate for the buoyancy gained. If you start out with the wing empty, or nearly so, by the end you will be perhaps four pounds light with no way to adjust. Your wing/BC needs to be nearly empty at the end of the dive, not at the beginning with a full tank. I go a little head down while hanging out at my safety stop and reach back to confirm there's only a little air in the bottom of my wing, but that it's not completely flat. Before there were BCs the only way of compensating was with lung volume, but the older tanks were smaller with less of a buoyancy swing to manage, and the weighting needed to be more precise.