Hehehe... a bit tricky... okay... one step at a time.
Vacuum modulator helps tranny shift smoother at all RPM... depending on the
available vacuum pressure, it determines how firm (low vacuum) or how soft
(strong vacuum) to shift. The shifting firmness is necessary when
accelerating or otherwise the friction plates inside the tranny will burn
out... sort of like manual tranny...
The modulating tranny cable tells the tranny how hard you are accelerating
so it adjust when the tranny should shift gear. When driving normally,
tranny see light throttle and will proceed with quicker shifting... meaning
you willl see 4th gear very soon. When you accelerate hard, it will tell
tranny to hold each gear longer so you can accelerate faster.
So by adjusting the cable tighter, it kinda fools the tranny that you are
accelerating harder than normal... and hold gear longer. Then again, the
adjustment goes out of whack as the cable stretches due to age, so it need
proper adjustment once in a while.
Now, as for heavy acceleration... tranny senses you stomped on your gas
pedal... so it tells itself that it has to downshift so you can drive the
bat our of hell... hehehe.
Lastly, when tranny age, the valve body will go bad due to leaking seals...
and the tranny will shift very funky and fast that no matter how tight you
adjusted the cable, it won't make a difference... then you have to decide
whether to do a valve body service or new tranny.