Well, I am a lady, and I am blonde, and not a math freak *grins* - let me try to explain it the way I understand it and the way I use it.
Variable voltage allows you to "feed" some higher voltage to your vaporizer. This causes more vapor and thus, is more satisfying. (Once I got used to that, I was no longer content with those batteries that deliver only 3.2 volts).
Start on the low voltage, like 3.2 volts. Then turn up the dial / twisting thing slowly (!). Vape a little with that, see how you like it. If you notice no difference at all, turn it up like 0.2 volts higher, and vape a little with that. And so on until you are satisfied with the output. That is what is meant by "finding your sweet spot" for that particular vaporizer. - And yes, it does make a difference. And it is very pleasant.
Caution:
do turn it up slowly please. Turning it up too fast can cause a burnt taste (very yucky), and turning it up much too fast / much too high can break the vaporizer (burn out the heating coil, no outside "breaking" visible. It is just "broken" then = no longer works)
Please excuse if I talk funny. All my vaping vocabulary is in German.
In Germany, we work with "rule of thumb" for the non-technically minded, for different vaporizers:
eGo goes up to about 3.7 volts
IMIST works well around 4 volts.
Vivi Nova (1.8 ohm head) works well at about 4 - 4.4 volts
This is all not rocket science
And if you turn up the volts carefully, you will find your sweet spot in no time.
Happy Vaping!
Edit:
About that Watt stuff, I have no idea, and I am not going to mess with it.
I was given the "rule of thumb" about the volts for different vaporizers over a year ago, when I bought my first VV batteries (the eGo Twist), and that works well for me. No idea about Watt, except for that there is some mathematical formula involved.
Nawww.. can't teach an old dog new tricks