Jimmy,
XIEDs are fuel mix enhancers that plug in between the O2 sensors and the ECM. They "trick" the HD module into running the fuel injectors slightly richer to offset the very lean factory settings programmed into the ECM. Please see "Nightrider.com" for more detailed info. The XIEDs won't replace a full-on tuner, but for the modest performance mods I've made at this point they work just fine.