Bill Wyman or reporters said : "Mick and Keith were not here. Bill was with Brian and Charlie, and Bill creates a riff. Mick and Keith arrive when Bill, Charlie and Brian played together. And with the riff of Bill Wyman, Keith and Mick would create the title Jumpin' Jack Flash. Bill wasn't credited".
Keith said : "The story of Bill Wyman is not true. I was in my house with Mick. I see my gardener Jack Dyer and I say "Jumpin' Jack". Mick adds: "Jumpin' Jack Flash".
In fact, the pronunciation of "Dyer" is a little bit similar to "Day". In reality, Keith loved the soul song "Before it's too late" of "Jackie Day" Keith said : "Jumpin' Jackie", "Jumpin' Jackie Day". Maybe Mick said "Before it's too late, yeah it's a gas".
Eventually, Keith decides to transforme the riff of Before it's too late in a rock one. They transform their joke "Jumpin' Jackie Day" in "Jumpin' Jack Flash". "Before it's too late" becomes "it's a gas". The riffs are easy to compare and third proof Keith was interested by the effect of saxophone at the end of the song, he imitates the effect on two new songs : Street fighting man and of course Jumpin' Jack Flash.
Of course, we have a real rock transformation between the soul title and the rock legend.
Another idea. In 1966, the title "Think" of the Rolling Stones is very different of the "Think" of James Brown, but the two songs have similarities. The two songs accentuates the word "think". I think the Rolling stones wanteds to do something a little bit similar to James Brown but in another direction.