Whatever this person told you is 100% false. The opto is a digital control, on/off as far as the pinball machine is concerned. There is no analog sensitivity to the flipper buttons that will send a varying voltage to your flipper coils.
However, you can produce a softer hit when you have a ball trapped. This is especially useful during multiball if you have a bunch of balls settling onto one raised flipper. If you can do it quick enough, you can de-energize the flipper so it drops ever so slightly and then re-energize it. This can help settle the balls out into a nice line if they are piled up ontop of each other, or even pass over to the other flipper with enough practice.