The first four of tai-chi method of power (Peng, Luo, Ji and An that I talked about in previous posts) relates to attacking forces (Jing) applied along the straight line joining one with one's opponent, and defending (deflecting) forces applied side-way or downward. The next next four tai-chi method of power relates to attacking and defending forces applied obliquely. And as before, I shall try to relate their applications to actual combat situations. Needless to say, I am not explaining things exhaustively, and I would certainly agree with "orthodox" tai-chi practitioners that I have not exhausted all aspects of the use of the eight tai-chi methods! It is more true for the four oblique methods which can involve more complicated applications.
In close contact situation, the use of Zhou (肘 - elbow) is common in most martial arts. In tai-chi form-pushing-hands practice Big Luo (大捋), a front-strike to the opponent's sternum is used. And its use is after one Ji attack has been neutralized by one's opponent's An defense. Being at close distant, one executes a Zhou (elbow) attack.
The usual question: How to use tai-chi Jing in executing an elbow strike? Answer: "Chi in Palm finding Chi in shoulder". Having said that, it also requires a highly flexible shoulder joint combined with a high degree of proficiency in muscles-as-one (肌肉如一), otherwise one's elbow strike can easily become too slow as well as too off-target! Therefore, the old saying "don't use elbow if you're not sure" has its merit.