Define the "path of acceleration" in more detail. Are you describing the rod tip, line path or casting hand?
I could never get the concept straight in my mind on "straight line path of the rod tip", this is just not reality,,,,we begin the forward cast with the rod tip low and to the rear, move the rod tip forward and then haul with the line hand,,,,this imparts additional load on the rod and it deflects (toward the water or lower) now we stop or SUAS the rod and the rod tip flips over to vertical. No way did that rod tip make a straight line path as most of the text books and instructions insist is the most efficient way to cast. The rod tip actually makes a concave path when we line haul, the haul does not use very much energy to speed up the line,,,,it loads (bends) the rod.
The method taught by Left Kreh is not universal or accepted by tournament casters or most of Europe. If you would visualize a graph with casting hand path (distance) on horizontal axis and acceleration on the vertical axis. If the casters rod hand moves say 36" overall from the begining of the forward cast to the stop,,,,,,then 34" is very slow acceleration and contributes to 10-20% on the acceleration graph axis,,,,,now the final 2" is the actual SUAS and contributes 80-90% of the acceleration,,,,,the line haul must occur in the final 2" of SUAS. The acceleration curve jumps to maximum and down to zero in the 2" of hand travel. If the wrist is not flexed durning the SUAS the loop will be very small,,,,,if you flip the wrist it will open the loop. I try to use my shoulder muscles to execute the SUAS with a single handed rod,,,,,,not the wrist as most trout FF.
One thought on the SUAS,,,the rod can be vertical,,,,45 degrees above horizontal or actually horizontal when the SUAS is executed. The line will follow the direction of the rod tip at the end of the SUAS. When we watch Lefty cast he executes a smooth fluid motion and it is difficult to separate the SUAS with the final "frogs hair" push on the thumb which drops the rod tip after the SUAS is completed,,,,this is done so the line does not crash into the rod tip (especially if the rod is stopped vertical) and it does open the loop a small amount.
I find that the majority of casters do not use the SUAS and if they actually do have a good positive stop,,,,their acceleration is fairly constant throughout the rod hand total path of travel.