Tzu-jan

Some neigong can be cultivated by exercise.
Others are more subtle and need the mind to be engaged in a particular way until the neigong is habit.

The more advanced neigong arise because existing neigong are in place.
With practice and experience they arise by themselves.

Tzu-jan concerns cannot be made to happen or directly cultivated.
They are a by-product of ingrained internal movement habits.

Connection

Connection is literally the process of uniting body parts.
It is the most basic and simplistic concern.
Instead of moving your arms via the shoulder or relying upon local muscle strength, you 'connect' the arms to the back and move via the torso and legs instead. Maintaining connection when performing different tasks is a major undertaking for the new student.
It is not enough to do a movement.
You must move in a whole-body manner.

The hardest part of this requirement is to remain relaxed but not floppy, flaccid or crumpled.

Whole-body

Neigong is concerned with whole-body movement.
This is distinct to how most people normally move in that every body part should be involved in every movement.
Accomplishing this is not easy.

As a child you possessed whole-body movement, but as an adult you have lost it from a lifetime of bodily misuse and bad habits.

Neigong qualities

Having gained a crude sense of connection, a student may feel like a robot.
This is not correct.

The aim is to move freely, comfortably - in an agile, supple, flowing way - without ever losing connection.
How is this accomplished?
By incorporating subtle yet sophisticated concerns into how you move.

These neigong qualities serve to unite the body in a manner that is not awkward or clumsy.
 

The body mechanics of the internal martial arts are significantly more sophisticated than those of the external martial arts. 

(Bruce Frantzis)

Principles

Internal principles of body use must be trained thoroughly and mindfully.
A verbal understanding is inadequate.

Tangible, functional application in your everyday life is the only way to truly understand the art.

Sustained power

In the internal arts, first and foremost, you never have a second where you don't have power. Regardless of whether you are yielding or attacking, there is no time when you lack power in your arms or anywhere else in your body. That's how the internal arts are done, and it has many practical benefits. For one, it prevents exhaustion.

 This issue of unbroken power is a pretty basic difference between internal and external martial arts.

 (Bruce Frantzis)