[trux]

For deep dives, the flexibility of the diaphragm (and the ribcage) is actually more important than the RV. Well, to certain extend the two aspects are related, but great diaphragm flexibility does not necessarily mean you will be able to exhale actively more to reduce your RV, while it will definitely help at deep dives when the diaphragm flexes passively.

Most freedivers use Uddiyana Bandha (and other similar yoga exercises), and packing stretches (positive and negative) to improve the elasticity of the diaphragm (and the ribcage).

EDIT: on the other hand, excessive (positive) packing is suspected that it may cause over-stretching of alveoli, with the consequence of loss of elasticity of the tissue, hence inability to shrink fully (increased RV), and consequent possible collapse of alveoli at low exhale volume. The collapse of the alveoli in depth would not be a big problem from mechanical point of view, because the alveoli gets inflated back during the ascend, but it may lead to pressure build in the enclosed cavities and to possible rupture of the alveoli tissue at re-inflating of the collapsed alveoli.

I am not sure if this is confirmed medically, or whether it is a pure speculation, but I heard multiple times the claim that heavy packers may have problems with RV.

Also an experienced osteopath may help with skeleton/musculature manipulations to unblock certain limitations and considerably help to increase VC and decrease the RV. When you look for an osteopath, you best search one that works with swimmers or athletes - the manipulation for unblocking pulmonary capacity is often used by them.