Fa Jin or fajin is a term that denotes an explosive use of power in an attack and in common usage, "Fa Jin" and "Fa Li" ("Li" meaning just "strength") tend to be used interchangeably. Not all fajin is done the same way, it should be noted, so seeing someone doing some sort of power-release and calling it fajin doesn't mean that it's using the same body-mechanics as other people do.
Once, while watching a somewhat comedic movie about Fong Sai Yuk, I saw one of the experts do a fajin with his sword that penetrated the armor of one of the bad guys. That potential usage of fajin piqued my curiosity, so the next time I met with an honestly skilled expert (Chen Xiaowang, in this case) I asked about that particular scene in the movie. Chen Xiaowang said that it could be done, but the power and the sword had to be in one finely-focused line of application so that the sword did not bend. And of course, to maximize power in any application this must be true. Practical fajin is applied in a narrow focus.
Recently, I watched a purported european expert in the Chen-style Taijiquan do a broadsword form and I was interested to see that he used one of those very light-gauge broadswords ("dao") that lets loose a nice rattly noise when you shake and thrust it in a supposed expression of fajin. If you ever watch a lot of people who do various "forms" for demonstration, you'll see that a lot of them use pretty flexible weapons because those too-flimsy-for-real-usage weapons display so well in forms demonstrations. If you watch very closely, you'll see that many of the forms demonstrators actually assist the shaking of the weapon with a sort of horizontal shake of the arm as they extend the weapon.
In other words, it's not really fajin, it's play-fajin. Real fajin that uses the dantien, 'suit', and jin uses very different body mechanics. The same fajin mechanics that apply to a sword apply to the hand or other parts of the body because a weapon is considered to be only the extension of the hand, in terms of body mechanics, jin, suit-control, and so on.
Hopefully, the next time you view someone performing in a contemporary wushu mode (as opposed to doing a form with the more rigid requirements of traditional wushu) and shaking and rattling to beat the band, you'll have a better idea of what is going on.