Hi P.
"Fa" means to send out, or issue. "Jin" is generally energy, or strength, but many people limit this definition to properly generated internal energy. "Fajin" is often associated with explosive releases of energy.
In my training, however, "fajin" is considered any time you release energy or send out strength that will affect your opponent. With this defintion, there are many types of fajin and many techniques that use fajin. Explosive releases are just one kind.
I guess, I should clarify what I mean by energy. When talking about martial arts, we are usually talking about mechanical energy.
Do we release "qi"? Probably, it is certainly involved. But "qi" is such a foreign concept and way of thinking that most people's understanding of "qi" is so vague or misdirected that it not helpful in learning Taijiquan or other internal martial arts.
FWIW.
good luck,
Richard Johnson
Chen Style Taijiquan Practical Method intensive training courses.
www.chenzhonghua.com