My thinking is that its best to find ways to cultivate/nurture aspects like Fa-jin. They may or may not come out in drills, sparring, real-life encounters/scraps etc. The best we can do is to build a structure for internal and external muscle memory combined with martial basics e.g. guard center-line, tactics, movements, timing, evasion, interception etc. In the end "Fa jin" is just a descriptive name to a combined set of qualities which is labelled as such fro simplicity - and does not describe the years of effort of development which produce this as a by-product i.e. Fa-jin will only come as a consequence of other work. For external MA folk used to sudden surges of energy, this may be a vehicle to gain interest, but at some point its probably for the best to get beyond this. Different IMA have different training tools to get there, but Fa jin is a consequence of the training not a goal IMHO.