Qi = energy
Jin = use of energy or force.
You use Jin in every movement. The focused use of Jin is sometimes called FaJin or "warrior/combative force"
In my system of hebei xing yi we differentiate between hei jin (dark jin) and an jin (bright jin). Dark jin penetrates or impales an opponent driving through their centre. An jin explodes on the surface or just under the surface. Most of our strikes are dark jin strikes, they displace and unbalance as well as hurt.
There are many other specialised ways of using jin. Many of them are just classifications or groupings of different ways in which you use energy (qi). The essential thing is that you have to do something to qi to make it into jin, the way in which you make jin determines what type of jin it is. E.g. spiral force = tsuan jin, expanding/contracting force = beng jin, frictional force = nian jin etc.
Before you can effectively use jin in xing yi you need a good structure and good grounding in the basics of the art. You can still make jin and you can still hurt people using xing yi without any real jin or internal force, however in this way it is still very much external in nature. As you get better things become more internally focused. this is an enormous subject and very difficult to achieve, i've really only just started to touch upon this myself and i probably know more of the theory than i can actaully apply. I'm still learning