The controllers clean up or reconstruct the AC signal, regulate it as to frequency and lower the voltage. My experience is that they provide a degree of improvement and clarity to the sound much as does the addition of a flywheel or a heavier platter (providing more flywheel). If you have a revealing system and a discerning listener, the improvements are obvious.
Having said all that, the speed of a belt driven TT may still drift from 33 1/3 because of aging bearing lubricant, an aging belt and/or needle drag. Other factors? Hence, many users welcomed the Roadrunner tachometer which measured average platter speed over a period of about 3 revolutions. When brilliantly mated to the electronic drive as was done with the Falcon and later the Eagle, the frequency would be adjusted slowly, but quickly enough as to be done right and not noticed by the listener. It is a "set it and forget it" system. (This differs from the instantaneous correction, as in Asian direct drives with relatively light platters, which, according to many people results in a deterioration of the sound.)
IMO nothing has approached the Roadrunner-Falcon/Eagle system which, unfortunately, is no longer available, and not even on the used market--for no one is parting with them. And just for the record, these were sold at a much lower total price than the competing VPI systems and IMO were built to a much higher engineering and physical standard. VPI has literally spent years trying to come up with something comparable, but with no apparent success.
As for this new Australian device noted above, despite the pretty page filled with hype, very little information is actually given regarding its actual use, particularly about its "tacho" or "strobe" system, nor about any incremental cost for them.
