You lost me some here, how can you have an average of an instantaneous value? Don't you need a sample from a set interval and take averages of those samples. I can see those samples being as often as milliseconds or even less apart. Otherwise the average of a single instantaneous value is that value divided by 1 = same value.
In an analog device (like an analog multiplier in this case) the speed of the disk is proportional to the integral of the instantaneous values over one cycle, or using a less precise term, the average of those values.
More precisely, the motion of the disk over a time interval is proportional to the integral of the voltage-current product over that time interval. Which when divided by the time interval gives the speed. The use of the word average is not limited to discrete digital samples.
If I time your car traveling one mile in 60 seconds, I do not have to take any individual measurements of your speed to do that, I just record the time at the start and the finish, giving me the average speed.
Sorry, got carried away lecturing.