Whether in a parade or backing out of the driveway who, "Lifts their foot OFF the brake while driving...." in order to decelerate? Greer definitely Placed his foot ON the brake when all hell started breaking loose.
I think he meant to say that Greer took his foot off the accelerator, not the brake.
The noted physicist Luis W. Alvarez came up with a siren-reaction theory, published in the
Journal of Physics in 1976.
"The heavy car decelerated suddenly for about 0.5 sec (10 frames),
centered at about frame 299, reducing its speed from about 12 mph
to about 8 mph. Since the car was certainly being operated in some
low gear ratio, the deceleration was no doubt caused by the driver
reducing his foot pressure on the accelerator pedal. ...
"suggest that the siren, from an escorting police vehicle behind the
President's car, had come a few seconds before the fatal shot. ...
"Everyone will recognize that such a reaction on the part of the driver
would be an unavoidable conditioned reflex; we all learn that when
we hear a siren suddenly turned on just behind our car, we lift our foot
from the accelerator pedal."