The thing about the dantian and reverse breathing is that those two things pull and manipulate the elastic tissues of the body. The pulling and manipulating and squeezing and 'beating and drumming' (in some qigongs) are meant to develop the functional qi.
Just to use a quick and very simple example (from which more complicated motions/breath can be extrapolated) I'll take two small excerpts from a video of Zhang Xue Xin when he was doing the Hun Yuan Qigong that Feng Zhiqiang devised. In these two very simple movements, Zhang is first tracing the contractile/Yin route on the inner sides of the limbs during his inhale and "pulling in" ... and then he is pushing along the outer parts of the limbs for the exhale and mild contraction along that extensor pathway.
So, what he is doing is working the tissues along those channels with the inhale and exhale and he may be visualizing a flow of qi along those routes in the direction of his hands. It's a sort of double-duty thing: a practical part and a mind part.
Remember that if you bend and pull your limbs inward (Close), the pulling parts are along the inner and undersides of limbs; when you extend and push your limbs outward, the extensor action (or Opening) is along the upper and outer surfaces of the limbs. In other words, the "qi flow" is inward pulling on the insides of the limbs with the inhale and the qi flow is outward along the outsides of the limbs as they are straightened (even if you're not moving the limb in the qigong).
The video:
