Fa Jing is externalization of power using full body. In tai chi demonstrations, it is usually done as pushing a compliant partner far away. In combat situation, what is the meaning of pushing people away and they can come back and fight with you again? In some martial art circle it is believed that Fa Jing can inflict inner damage, and in some legend it was said a master could be able to control whether or not internal damage being inflicted! Belief and legends are interesting. In combat, what is effective boils down to either a powerful punch (kick etc) or an effective throw. In modern combat sports (for example San Da), punching power is usually developed ultimately using a punching bag, like other combat sports. Fa Jing's uniqueness is now more associated with health and fitness training, and with a special edge that I shall explain.
Fa Jing is a general terms that include a number of ways to externalize one's power. Sometimes limited to a local tool (like fixing the elbow or the shoulder) for training of specific parts of the body, in the understanding, training specific parts can be added together to form a more complete training of the whole. There are whole body training methods too, like the regular (Fa Jing) push (actually called Dan Jing 彈勁) in which most people can see in demonstration. Dan Jing is not the best training tool for the whole body conditioning for health and fitness. But before I select one that I consider the best, let me tackle the question: How is the training objective being achieved?
Training objective is achieved by the bouncing back effect of arm-movements acting on the body. Each action is repeated a large number of time (20-100). As a result, chi power will be accumulated easily and effective (without losing chi energy). When an obstacle is not there, the bounce back power should be made to push back too the body which should (therefore) be maintained in a relaxed state. The general guideline is that such training is for students with stronger physique which means before seventy and being in good health. So it is more or less applicable to most students.
Since the bouncing back effect is so important, the best training method is the "Shaking Jing 抖勁". For simplicity, it can be done is a standard combat stance pose. You simply shake your arms front-and-back when in stance. The most important point is to manage the chi generation and accumulation process. If you have a good foundation in chi training, you should no problem in generating such chi sensation in one lesson. And you can appreciate the effectiveness of the cumulating training after just a few sessions. If you cannot, probably you need some time to build up your foundation. And you can do it properly, you will sweat in no time, and your body really feels good!