I heard someone say it, but I just assumed they meant “snapping their fingers” and it’s a regional difference.
I can’t imagine anyone saying “cracking your fingers” and not meaning the same thing as “cracking your knuckles”.
I didn’t even think of the knuckles! 
Knuckles was how I took it, too.
Was it someone you know in person? Can you ask them to demonstrate, without giving them any hints about what you already think it means?
No, it was a video. I’ll reach out on Twitter, though.
You can certainly crack the joints in your fingers that aren’t the knuckles. And that is how I would assume it was meant.
I crack my knuckles and also the joints in my fingers.
I’m not sure what you’re referring to here. To me, all three joints in my fingers are called knuckles. Are you talking about the proximal end of the metacarpals in the palm? Is that something you can crack?
In my experience “crack your fingers” is a perfectly normal and common expression that is used exactly as “crack your knuckles.” I personally say “fingers.” I don’t think there’s any great mystery as to what is meant.
Well, I learned something today. I thought only the (looking it up) metacarpophalangeal joint or “first knuckle” were called knuckles.
I wouldn’t claim that you’re definitively wrong - it’s not a technical term, after all. It may be that other people use it your way. I see that at least one dictionary definition is suggestive of that, saying that it “especially” means the first knuckle.