I only know of the variable wattage ones. I have the Smoketech "The Kick". You put it in the battery tube on top of the battery on top of the battery and adjust the wattage up or down, trial and error, with a small screw driver until you get the vape like you like it. After that you can just forget about it. Besides giving you a little wattage boost, it makes you have a more consistent vape by boosting the wattage as the battery voltage drops. It will also make different resistance heads seem the same. I went back and used up my heads that I had bought that were to high resistance after I got mine. Your batteries will last longer because you can drain them as low, which is hard on them. It works as a fuse too. If you have a short it will blow before the battery. They are only good for 5-12 watts. I think the Kick 2 goes up to 15 watts.