How does rooting work in Tai Chi?

In the Tai Chi Classic is says:

The jin should be
rooted in the feet,
generated from the legs,
controlled by the waist,
and expressed through the fingers.

This does present something of a dilemma – how can we both be rooted in the feet but also controlled by the waist, let alone also generated by the legs?

In the Treatise on Tai Chi Chuan, the classic says “Sink the chi to the dantien.”

This sinking is related to the pulling in action of the muscle tendon channels that are usually associated with the contraction (closing) phase of opening and closing – usually while breathing in. The breath sinks to the dantien area, and combined with correct mental focus, this should make the dantien area feel full as you breathe in.

This pressurised feeling in the dantien is also the pressurised feeling at soul of the foot. You could think of it as squeezing the pressure from the dantien down to the foot, but you really don’t need to do that, as it should be instantaneous, since they are the same thing.

With the exhale, the pressure pushes up, through the legs, up the back and out to the extremities including the head. It’s a continual cycle of store and release.

You could call this the internal side of Tai Chi, but really, it’s just the way Tai Chi works, rather than a particular side of it.