Expert students must move from a connection/leverage-based structural framework/use of the body onto an energy-based functionality.
This is a big step.
Li is easy, jing is not.
In order to facilitate vibration and effective reeling the student must:

1. Internalise
2. Free the joints
3. Stop stretching
4. Rely upon soft tissues
5. Reduce the frame size
6. Use a far smaller circle
7. Express sung and folding
8. Make the stances smaller and higher
9. Arcs and circles are largely replaced by spirals
10. Bring the hands closer to the body and not as far away

The delivery mechanism becomes less overt, more subtle.
The outcome/effect is quite different.
Fa jing is possible.