I don't understand what fire hot and using mean or RMS mean. It'd be great if you could elaborate.
Running hot: the battery inside the device outputs a certain voltage depending on its charge state (around 4.2v at full charge, down to around 3-ish volts when it needs to be recharged.) Regulated devices use "buck" and/or "boost" circuitry to raise or lower this voltage to whatever device output is required for the user-selected voltage or wattage setting. The iStick 20-watt does not have "buck" circuitry, it can only "boost" the voltage higher than the battery charge but can't lower it any. Therefore if the battery is at, let's say 4.0 volts, for example, and you have a 1.2-ohm atomizer attached, you won't be able to fire it at anything
less than 13.3 watts. If you like your vape at 10 watts, too bad, you can't get that low until the battery drains down to about 3.4 volts. The 30-watt (and most, if not all
other iStick models besides the original 20-watt) have boost
and buck circuitry, and so they can "step-down" the voltage as well as increase it, to fire a wider range of wattages on any atomizer you choose.
Mean/RMS: Some devices have a "flat signal" where they output a specified voltage at a constant rate. Others use what's called "pulse-width modulation" or "PWM" where they fire at a higher voltage, but flip the power off and on really fast (many many times each second) to simulate a lower power level. They vary the "percieved" power level by varying the amount of time the pulse is "on" versus how much it is "off" -- by "modulating" the "width" of the "pulse" hence "pulse-width modulation."
To determine what the "percieved" power level is, you have to do some math, on the amount of time the pulse is on and what its voltage is. There are a couple of different ways of measuring this. One easy way is to just calculate the "mean" (another word for "average") voltage over time and use that to calculate the power level in watts. This way is easy, but it's not very accurate. Another, better, harder way is the RMS or "root-mean-squared" formula. This is where you calculate the
square of the voltage at any given instant, take the average of those squares, and then take the square-root of that average. The root of the mean of the square, or "root-mean-square." This method better represents the power level because power, or wattage is not a direct function of voltage, but of voltage-squared. By using the RMS formula you will get a result that more closely resembles a "flat signal" at the selected wattage, compared to the easier, "mean" calculation.