Mind

Yang Cheng Fu said "Use mind not force" and this one statement holds the key to understanding the difference between internal and external.
Intention requires considerable presence and awareness.
The student must have a calm, clear mind; focussed on the here and now.

The mind is used to create energetic outcomes within the body.
For example: a student seeks to 'sink' and 'root'.
A beginner may accomplish this by dropping deep into the hips, bending the knees and bearing the weight down.
Such a method would be fine in most martial arts, but in what way is it internal?

The physical action needs to be slight. No deep bending. No bearing down.
Use your mind instead.
If this seems difficult to you or unlikely, it reveals the fact that your training remains largely 'external'.