DiscoverTaiji, there is an article about 3 key elements of good taijiquan: Timing, Placement and Power.
An excerpt is below. The full article may be read here.
Timing, Placement and Power
An article by Adam Mizner
When I am teaching classes or workshops on taijiquan I always emphasize the three principles of timing, placement and power.
These three skills are not only fundamental to acquiring real world
taijiquan skills but are fundamental to the successful application of
any martial arts technique.
Bruce Lee and other famous martial artists often talk about speed as
one of the most important attributes of a successful martial artist.
This is not untrue, though I would say that timing is more important
than mere speed. It is certainly possible to miss the mark because one
arrives too early or is too fast.
Understanding this we should strive to master timing rather than just
speed. When we arrive “on time” in this way, our opponent is where we
perceived him to be and our technique is neither early nor late. In tai
chi chuan this ‘correct time’ is when the opponent has “fallen to
emptiness”, he is off balance and frozen or double heavy. This is the
right time to attack and finish the confrontation.
Many attacks delivered with the wrong timing are not as effective as
one that is delivered on time, whether it be delivered fast or slow.