He said "the left turn" not the curve. He had just begun to relax the pressure on the steering wheel when he heard the first shot. The car's front wheels would then gradually go from turning to straightening up.
Mrs. Johnson said they were still rounding the curve. Lyndon Johnson wrote in his 1971 memoir: "Just after our car made the left turn at the top of Elm, I was startled by an explosion." Youngblood testified: "As we were beginning to go down this incline, all of a sudden there was an explosive noise."
That is your editorialzing. One straightens up after finishing the turn. Do you actually drive a car, Jerry? Lady Bird did not use the word "still". She said they were "rounding a curve, going down a hill" (5 H 565). You are suggesting that means "making a 120 degree turn"?
In any event, your suggestion does not fit with what the occupants of the VP security car said. SA Kivett said (18 H 778): "As the motorcade was approximately i/3 of the way to the underpass, traveling between 10 and 15 miles per hour, I heard a loud noise - - - someone hollered "What was that?". He said that their car had "just turned the corner", as did driver Joe Rich, Clifton Carter, SA Johns and SA Taylor. Clifton Carter said "7 H 474: "our car had just made the lefthand turn off Houston onto Elm Street and was right along side of the Texas School Book Depository Building". None of that fits with the position of the VP security car at z160, which is just entering the intersection at that time:
No unusual movements immediately after Z198.
Youngblood's statement is consistent with the movements seen after the car emerges from behind the Stemmons sign at z223 and after. The point he was making was that he looked around after the first shot and when he looked at the President's car and the Security car he saw abnormal actions. He had been watching from that position during the entire motorcade so he would have a good idea of what was and what wasn't normal.
It is also worth noting how Youngblood and Lyndon Johnson say he reacted to that first shot. He told the Johnsons to get down and stepped/"vaulted" over the seat and got on top of the VP. We can see from Altgens' no. 6 photo that he is facing the back and is in the process of climbing over the seatback by z256 but is not yet on top of the VP: