I have heard it said that every time someone strikes they have a vulnerability. I did not realize how true this was until the other day when I was doing push hands in the park. Every time my partner or I had an open door or gate we would remind each other with a friendly thump. But it got to the point that nearly every time there was an opening we got hit. Being jabbed in the side over and over is an educational experience.
It soon became clear that there was only one viable defense against many of the strikes, when the opponent moves to strike in the open door, you also move and arrive first. The best defense is often a good offense and this method was the only consistent way to avoid getting hit at specific points in the game. While aggressive techniques are often downplayed in taiji, when used properly they really command the situation and maintain initiative. To get caught up in defending began to result in failure over and over, instead the only viable defense for me was to quit responding to attacks by way of defense and respond with offense. The only way to do this was to stop being passive, instead of reacting I had to act like a cat, I got ready to pounce and instead of merely acting on my own I waited for the opening of the attack to present itself.
Every attack has a weak spot, a vulnerability, an open gate. This is yin and yang in a way, if the duifang hits high you hit low, if they hit left you hit right. I understand now why it is said that my opponent moves first and I arrive first, this is part of the brilliance of taijiquan, and without this understanding there is little that can be done against a skilled player. At least this has been my experience.