Centering, like other combat training exercises in Taijiquan, should not be practiced as though it were an end in itself. Always keep in mind that it is a step in a progression to building skills in Taijiquan fighting. When I participate in or watch centering, I often see gross violations of Taijiquan principles, which will become dangerous bad habits if allowed to continue into subsequent levels of combat training.
One common violation is the departure from the principle of centered and upright (no tilting, no leaning). Here the partner contorts himself forward or backward to avoid losing his balance and “losing” the centering “contest.” These contortions expose the groin when bending backwards and expose the back of the skull when tilting forwards. Very bad habits indeed.
Another very common violation is resisting, the use of force against force creating mutual stagnation and turning centering into Sumo. The skilled practitioner will quickly resolve mutual stagnation by turn Yang to Yin, but this if difficult to accomplish when the rules do not allow stepping.
Butting, the use of excessive force as noted by Yang Ban Huo, is another common mistake. When a partner attacks with excessive force–in effect throwing his center into the attack–a skilled practitioner can send him stumbling across the room by directing his attack to the side, but practice is necessary to develop effective reaction and timing.
Finally, there is the failure to give up ego and accept Cheng, Man-ching’s principle of “invest in loss.” Don’t look at centering as competition. Look at it as training. You are supposed to make yourself vulnerable so that you can increase your skills. Thus, the suggestion to stand high rather than low. If you consistently “win” at centering–and especially if you resort to violation of essential Taijiquan combat principles to “win”–you learn nothing except that your partner is not skilled enough to make you improve.