Nothing in the world
is as soft and yielding as water.
Yet for dissolving the hard and inflexible,
nothing can surpass it.

The soft overcomes the hard;
the gentle overcomes the rigid.
Everyone knows this is true,
but few can put it into practice.

This quote captures the essence of taiji softness.
Only hardness can shatter, only firmness can be broken.

Yang Chen-Fu said: "One must distinguish the pure from the motley. Many practice taiji nowadays but it is not the real taiji. The real has a different taste, and is easily distinguished. With real taiji, your arm is like iron wrapped with cotton. It is soft and yet feels heavy to someone trying to support it. You can feel this in push-hands practice. When you touch an opponent, your hands are soft and light but he cannot get rid of them. When you attack, it is like a bullet penetrating cleanly and sharply (gan cui), yet without using any force. When he is pushed ten feet away, he feels a little movement, but no strength and no pain. In touching him, you don't grab him. Instead you lightly adhere to him so that he can't escape. Soon his two arms become so sore he can't stand it. This is real taiji. If you use force, you may move him. But it will not be clean and sharp. If he tries to use force to hold or control you, it is like trying to catch the wind or shadows. Everywhere is EMPTY. It can be likened to walking on gourds on the water. You cannot get to where the substantial is. Put simply, the real taiji is marvelous."

I was very much inspired in reading this material:

http://0801.nccdn.net/1_5/008/335/23d/LiYaxuansExplanatoryNotesonTaiChiPushHands.pdf

Some excellent quotes are as follows:

When students begin learning push hands, they are usually afraid to relax completely, for fear that their partner or opponents will take advantage of their softness to suddenly launch a strong attack. What these beginners haven’t yet understood is that if you can give yourself up entirely to relaxation, an amazing power of ‘sensitive emptiness’ naturally emerges that allows you to easily handle anything that comes up. This continues to evolve until it reaches an extreme degree light relaxed power so that your offense becomes unstoppable and your defense is also an attack. Any position you assume is superior to the opponent and you come to totally dominate whoever you work with without any problem at all...

You should approach push hands with a light and alert spirit. Though you may have some combative skills don’t use them against him. Though you may have some techniques, don’t use those either. You must use your spirit to neutralize and counter. to make him feel he’s chasing the wind and grasping at shadows, as though he has nowhere solid to stand, as though there’s nowhere to run, nowhere to hide, let him defeat himself with his own actions, then he’ll be easily overcome. If you try to use techniques to block or counter his motions and attacks, he’ll be able to forestall your application of whole-body energy and all your efforts will be clumsy and futile.

Pay attention to this...

When you first touch hands in ordinary practice, you need to use a very light touch to feel him out, to follow along with him, responding and adapting to all his changes. You mustn’t pose even the slightest resistance...

You body is completely soft and

pliable, appearing to be boneless. No matter how your partner comes at you, you seem to be

able to absorb or redirect everything, extending and retracting at will, using invisible changes to

neutralize him...

Before you actually make physical contact, assume an attitude of emptiness. With that

established you can very lightly touch your partner’s hand. Once you have touched him, you

must not move of your own volition. Think of this phase strictly as a means of training your

sensitivity and awareness. Then you use your superior emptiness, awareness, perception, and

sensitivity to engage your spirit in order to neutralize even his slightest intent towards movement

against you, following and adhering without gaps or resistance...

I've found this commentary to be among the best i have ever read.

two as one

to stick
to follow
to lead
to flow

can the elbow strike the forearm?

no, because though distinct the elbow and the forearm are one
and though it cannot strike
what control the elbow has over the forearm!

the elbow is the root
to the trunk of the forearm

if you become the root of another
though distinct you will be one
follow and lead

Taijiquan is like this

if the forearm is severed from the elbow
the two disconnected
this is a disadvantage

if you are separated
from the opposing force
you are like an elbow
without a forearm

separation
unity
these things are not to be taken lightly

a single perfected motion
is more profound
than ten thousand motions
performed well

a single well placed step
is more secure
than ten thousand steps
eagerly made

neither rushing
nor hesitating
the steady way
is sure and true