The BarbTech fraction is what proportion of techs civilizations know once they transition from barbarian cities to proper civilizations. The barbarian faction itself tends to keep pace just behind the highest tech player by getting techs for free automatically and this is controlled by something different.
This is most apparent a while after discovering mountaineering for instance - animals and other barb units will start moving onto mountains even if none of the AI civs are even close to having mountaineering yet.
So, if you set it at 0.0, what I think would happen (if I understand it correctly) is that once a barbarian city has reached the necessary size, and you haven't reached the maximum number of civs yet, it will transition into a proper civ, which will have absolutely no technologies. This can be bad because it means a newly formed civ laden with clubmen could be easily conquered soon after forming by barbarians using heavy swordsmen or something.
So yeah, BarbTech controls something entirely different from what it sounds like. So just set it to 1. It's not like a newly formed civ could possibly build up the requisite infrastructure to produce stealth bombers or something in any meaningful amount of time. Unless you have permanent alliances on and they form an alliance with an old established civ that's well behind you on tech. Then it could be startling when the older civ's stack of doom of 200+ flailmen and siege rams upgrades to rocket artillery and modern infantry.