Ni and Ti are our introverted/internalized functions. It's how we make decisions, especially considering we're Ni dominant. Fe is what we use to bring forth those decisions to the rest of the world. It tells us the most likely ways people will react, which allows us to adjust our decisions along those lines. (Generally so our decisions are more acceptable towards others, but sometimes to be antagonistic on purpose.)
It's types that have Fi that will make decisions based on their "heart", because that's one of their internal/introverted functions. You could say that rather than go with our "heart", we're known to go with our "gut" instead. Ni/Ti brings forth a lot of hunches. Feel free to correct me if I'm wrong, but it's my understanding that the introverted functions of the types are the decision making ones, while the extroverted functions mold those decisions so they can be brought forth.
An INFP, for example, might be talking to someone about a cause they happen to support. They'll support that cause because of Fi. Because their "heart" tells them to for whatever reason. But you'll see them explain the cause using Te. They'll point out facts if they have them, that sort of thing. It'll come out in a fairly logical manner.
Whereas myself, an INFJ, will come up with all sorts of logical reasons on the inside as to why I might be in support of the very same cause, but I'll be using Fe to explain to another person my support of it. It'll come out as more personal. Take a war, for instance. I might find out how much money, and lives, have been wasted on a war. But I won't necessarily list those facts when I explain my stance. I'll probably say things like "violence only begets more violence" or "an eye for an eye only results in two blind people" instead.
It's late and I'm kind of rambling, but I hope that makes sense.
