The first version was AFM, active fuel management, which switched between eight cylinders and four cylinders.

DFM, dynamic fuel management, is the evolution of AFM. DFM is, well, more dynamic. Not sure what the minimum number of cylinders is but it can change up the number of cylinders and the firing order in a much more sophisticated way than AFM could.

At this point I get in over my head discussing it, but apparently it's very hard on the lifters and maybe even valves for the engines it's deployed in.

Range Technologies makes a unit that you plug into your OBDC port that disables AFM/DFM, so your engine is a full-time 8-cylinder engine.

It doesn't change the programming, and therefore doesn't void your warranty. It's easily added and removed, so you can remove it when you have to have the vehicle serviced. The best part is that these units are a little less than $200.

The reviews say that power delivery becomes much smoother and more consistent. AFM and DFM are fuel economy developments, but apparently they don't do very much except vastly shorten the life of your engine, and deactivating them results in roughly the same gas mileage.