Mods (Advanced Electronic Cigarettes) come in two basic types, unregulated and regulated. The iStick is a regulated mod.
On an unregulated mod the power from the battery is supplied directly to the atomizer (Coil, head, etc.). There is no way to control the output power of these devices. These devices will also get somewhat weaker as the battery drains and the voltage drops.
On a regulated device, like the iStick, an onboard computer controls the power supplied to atomizer. The power supplied to the atomizer is independent of the battery voltage. Regulated mods typically come in two types, variable voltage or variable wattage. Some regulated mods, like the iStick, can do both variable voltage and variable wattage. These two modes are mutually exclusive, meaning the mod is either in variable voltage or variable wattage mode, but it can NOT be in both modes at the same time.
In variable voltage mode the iStick will output whatever voltage you set. It won't care what the atomizer resistance is, it will just always supply that voltage.
In variable wattage mode you set the iStick to a certain wattage you prefer. The iStick will automatically detect the resistance of your atomizer and calculate the correct voltage to output based on this information. If you change atomizers the iStick will automatically readjust the voltage based on the new atomizer's resistance.
While the iStick is adjustable and somewhat automatic, it's still up to you to select a wattage or voltage suitable for the type of atomizer you're using. Different atomizers and coils (coil heads) are designed to handle different levels of power, so the best method is to start at the bottom of the suggested power range and slowly adjust the power up to achieve a satisfying vape.