Echo.

"Flash"is basically "Satisfaction" in reverse. Nearly all of these riffs are closely related. But if someone said: “You can play only one of your riffs ever again,” I’d say: “OK, give me 'Flash'.” I love "Satisfaction" dearly and everything, but those chords are pretty much a de rigueur course as far as songwriting goes. But "Flash" is particularly interesting. “It’s al-l-l-l-l right now.” It’s almost Arabic or very old, archaic, classical, the chord setups you could only hear in Gregorian chants or something like that. And it’s that weird mixture of your actual rock’n’roll and at the same time this weird echo of very, very ancient music that you don’t even know. It’s much older than I am, and that’s unbelievable! It’s like a recall of something, and I don’t know where it came from.  

Keith Richards