Standard Deviation Demystified in Power Pivot

Two weeks ago I posted about a particular student's knack for making me learn new stuff, specifically the "bubble up ranks" method. (Ooh, I forgot to circle back and cover the "bubble up exceptions" part - making a note of that now). Anyway, he also pressed me to teach STDDEVX(). That's a first. "No worries," I said, "it's just another X function - but only you will know if the numbers are correct, because I don't know what Standard Deviation truly means."