This particular statement by John Chism exactly matches to what Kellerman testifies to in his WC testimony about turning after the first shot and before the second, the head shot.
"And the two men in the front seat, I don't know who they were, looked back, and just about the time they looked back, the second shot was fired"
For me, Chism's observation about the headshot is validated because it is confirmed by the Z-film:
Zapruder thought there was only two shots.
Mr. ZAPRUDER - Well, as the car came in line almost--I believe it was almost in line. I was standing up here and I was shooting through a telephoto lens, which is a zoom lens and as it reached about--I imagine it was around here--
I heard the first shot and I saw the President lean over and grab himself like this (holding his left chest area).Mr. LIEBELER - Grab himself on the front of his chest?
Mr. ZAPRUDER - Right---something like that. In other words, he was sitting like this and waving and then after the shot he just went like that.
Mr. LIEBELER - He was sitting upright in the car and you heard the shot and you saw the President slump over?
Mr. ZAPRUDER - Leaning--leaning toward the side of Jacqueline. For a moment I thought it was, you know, like you say, "Oh, he got me," when you hear a shot--you've heard these expressions and then I saw---I don't believe the President is going to make jokes like this, but before I had a chance to organize my mind,
I heard a second shot and then I saw his head opened up and the blood and everything came out and I started--I can hardly talk about it [ the witness crying].
Mr. ZAPRUDER -
I thought I heard two, it could be three, because to my estimation I thought he was hit on the second--I really don't know. The whole thing that has been transpiring--it was very upsetting and as you see I got a little better all the time and this came up again and it to me looked like the second shot, but I don't know. I never even heard a third shot.
Mr. LIEBELER - You didn't hear any shot after you saw him hit?
Mr. ZAPRUDER -
I heard the second--after the first shot--I saw him leaning over and after the second shot--it's possible after what I saw, you know, then I started yelling,
The Willis 5 photo with JFK waving to the crowd in the area of Newman, Chisms, and the secretaries Calvery, Hicks and Westbrook at Z202+.