Rev matching on a MT is one of the most instant gratification exercises if successful!
With enough persistence, most MT drivers can learn to do it.
Let's accept that a good rev match downshift change takes about 0.5-0.75 seconds. De-clutch, while foot on the brake, nudge gas pedal while still on the brake and at the peak of the nudge, downshift and let clutch go. Vooorm, aaah!...
You can't stick your foot in there at the right time and for the correct depth of gas pedal in less than 1/10th of a second that the PDK takes for the downshift. It is beyond human reaction times. However prepared, anticipating or jumping the gun you might want to be.
The computer though is perfect! You downshift and you see the needle in its new position, not wavering, ready to follow your lead in this dance. You double downshift and you approach an aural nirvana... I heard someone had a triple downshift
done by the PDK itself, not with the paddles. I'd like to experience this one day...
It means you are hitting an apex at the limit of the brakes with your hair on fire!