I know I'm new to this and will probably catch some "you need to read more" flak but humor me if you will.
Could you elaborate on the buck you were talking about. I'm using Efest IMR 18650 3.7v 2500mah battery.
There is a buck circuit on some mods that allows the mod to regulate power down. IE if you are vaping at 3v on a battery with a 3.8v charge, you will get the 3v. If no buck circuit is in place then you set to 3v (or equivalent wattage for the ohm coil you are running.. ohms law READ IT lol) then the mod will just dump the batteries voltage (3.8v for the example) despite your setting.
All regulated mods have a boost circuit, which allows it to regulate voltage up.