The DKFH just supplies the sounds of the drums. Dev (and whoever else uses the program) must create the MIDI track which will be run through DKFH. This can be done with a program like Guitar Pro or tabIt and most likely heaps of others. In cubase (Dev's DAW of choice), you put in the MIDI track. By itself, MIDI information is not sound, so you have to run it through something to provide the sounds. That's where DKFH comes in, and magic happens. DKFH also provides some different options like different snare sounds and kick sounds, etc.
Just to elaborate a touch, there is a "general MIDI standard" which makes it easier for everything to work together. Basically, instruments have their own number (30 for "distortion guitar" and 00 for drums, I believe. There are lots and lots more), then you get to doing the notes. For drums, each drum/cymbal/percussion-thingy has a number. 35 is a kick drum, 40 is a snare, 52 is a china cymbal, etc. So you can arrange all of these together (one number for each time you want that note/drum/cymbal hit) and then send that information to Cubase, which is where the magic happens. MIDI also has the length of notes built into it, so you don't end up with hundreds of drum hits happening all at once.
Whew! I hope that helped a bit. I know it's kinda hard to understand (or not, and I'm just a bit slow), especially for someone who doesn't "need" to know this stuff to get it working.
P.S. I thought that Dev used EZDrummer with the DKFH EZX? I don't know where I read that. It definitely sounds like that EZX's samples. If nobody else knows wtf I'm talking about, then don't worry about it.
