Tai chi was designed to make your body an effective conduit for the use of kinetic energy.
If your body is stiff and tense, you will not be able to utilise jing.
It is necessary to be soft and loose, sensitive and open.
This does not mean flaccid.
The groundpath must be present constantly, otherwise you cannot transfer kinetic energy from your body to another.

If you are stiff and unyielding, disconnected or flaccid - this transmission will be unsuccessful.

Confused about yielding?

If taoism is the art of adjusting to life, then tai chi is the art of adjusting to the opponent.
This process of adjustment is what yielding is about.
Balancing, sensitivity, change.

Yielding is concerned with not opposing force, making space... and then counter-attacking.

Having made space, you must incapacitate your attacker. Yielding is only half of the requirement.
Unless you neutralise the attacker, they will continue to assault you.
Step-in decisively and finish-off the attacker.

The internal way of using strength has some basic considerations:

1. Never employ force against force; always yield to strength
2. No more than 4 ounces of pressure should be exerted upon your body or expressed by you
3. Each movement should be a whole-body movement
4. Unite internally using neigong yet remain soft, pliable and yielding
5. You can transmit strength via groundpath
6. Intention can unite mind and body into one focussed unit

People read these points and feel dissatisfied, as though some crucial part was missing.
You must remain calm and composed, relaxed and easy. 

This applies particularly to wannabe lineage students: 

I do not enlighten those who are not eager to learn, nor arouse those who are not quick to give an explanation themselves.

If I have presented one corner of the square and they cannot come back to me with the other three, I should not go over the points again.

(Confucius)

I think that bagua is fantastic. I see it as an honour to be taught something so detailed and deadly. Every new starter [including me] has different ideas about self defence and martial arts. This exceeds all expectations you could possibly come up with.

 Learning baguazhang at this stage in my tai chi experience is unexpected. I am getting to see aspects of shuai jiao and chin na that would only be possible much further into the tai chi syllabus. It can be rough depending on what is being taught, but it is never over the top. Pain within an application usually comes from being tense.

 (Barnaby Baron)

Internal skill is subtle.
It takes decades of time, understanding and training to cultivate: this is why so many people go astray.

The visible outward signs are small. Most of the work takes place within the body. The movements are smaller, less obvious.

As the student's skill improves, the physicality of the tai chi diminishes.
The frame serves to supplement the mind.
A more subtle physical expression is now possible.

Partner drills and form application teach the student how to minimalise their movements.
Balance, timing, structure, softness and mind combine to create the desired outcome: a twitch instead of an arc.

It is important to rest.

One of my own follies is over-training: my training reaches a peak where it feels fantastic overall, but I start having problems: tiredness, stiff neck, migraine...
These are signs that I am doing too much.

I pare my routine back to the absolute essentials, and give my body time to recover.
My energy returns and I am ok.

This sounds easy enough but it is hard to let-go of the extended training, hard to do less.
The urge to hold-on is strong, but also harmful.

When I was 25 my doctor told me to choose whether I was going to focus upon kung fu, yoga, running or cycling.
I was doing all 4 activities simultaneously and pushing it.
The doctor said that you cannot train like an olympic athlete every day without it taking a heavy toll.

I chose the kung fu because it is sustainable and mild on the body.
But only if the training is moderate...

It can be quite a shock for a student to find out that their form is actually a mess.
The form may look aesthetically pleasing, yet under pressure it falls apart.

Taoism advocates eating the fruit, not the flower.
The form must be functional, effective and comfortable.
If your structure and movements fail in combat, what exactly are you training when you practice your form?

Locke: What do you suppose is in that cocoon, Charlie?

 Charlie: I don't know. A - a - a butterfly, I guess.

 Locke: No, it's much more beautiful than that. That's a moth cocoon. It's ironic - butterflies get all the attention, but moths, they spin silk. They're stronger. They're faster.

 Charlie: That's wonderful, but...

 Locke: You see this little hole? This moth's just about to emerge. It's in there right now, struggling. It's digging it's way through the thick hide of the cocoon. Now, I could help it - take my knife, gently widen the opening, and the moth would be free - but it would be too weak to survive. Struggle is nature's way of strengthening it.

 (Lost)

The 'internal arts' are so-called because the focus is within.
You are required to feel rather than do.
Outward movement must reflect the inner condition and should stem from what is happening internally.
This sounds difficult until you consider it further.

Every movement made by the human body begins under the skin; nerves activate muscles and muscles move the bones.
There is nothing special about this; it is the normal process.
Tai chi simply reconsiders the way in which the movement is generated; it explores the how. 

External bad habits:

1. Force against force
2. More than 4 ounces of pressure exerted by you or expressed by you
3. Localised arm  and shoulder movement
4. Deep, long or wide stances
5. Fixed legs - disconnected upper & lower
6. Tensed muscles
7. Over-emphasis of the waist
8. Incorrect use of the pelvis and hips
9. Pushing upon impact
10. Aggression

These will all perpetuate an external approach to tai chi.

The horse stance is not a combat stance.
It is a training method designed to create greater postural stability and build-up the leg muscles.
There is nothing 'internal' about it.

Sets are derived from the form.
They use movements from the form against an attacker.
Therefore, they also serve to test your skill with form. If your form is imprecise and sloppy, your application will not work.

A combat set provides an opportunity for a student to really tidy-up their form and gain insight into how the form can be used.
Errors that may seem unimportant in solo practice may prove disastrous in partner work.