I have two ways to desulfate or equalize batteries.
One is the equalization stage on the IOTA, which does the process that probably has the least aggressive. This is to raise the voltage above bulk (say 15V to 16V) for a short period and limit current (generally said to be 5% of capacity at most). I have a group 27 AGM that's been on this charger almost 24/7 for about 10 years now I guess. I can't inspect the plates to know for sure but the battery still holds a good charge and has plenty of capacity. But it's not heavily used, mainly it's what runs my ham radio station and fridge when it's not in the truck, so it mostly just floats since we don't lose power much. It's possible that a battery that floats in its happy place would never sulfate extensively anyway.
I also have a stand-alone desulfation device, currently using a Power Pulse, that implements the technique described by dwh, which is to resonate the plates and is said to vibrate the plates within the cell. This is claimed to 'shake' the ions up and break down the crystalline build-up. There are various circuits you can look up but this one seems to be a variation that runs at about 25KHz with short rise time (in the tens of μs) positive going pulse and slow fall times.
You get opinions ranging from they are complete bunk to the best things ever. I continue to run desulfations, it seems to be helping (my Optima in the old truck was 8 years old) or at least not harming anything. Now both of these are running continuously. In the case of the Power Pulse it literally is always on running all the time from the battery power itself and the IOTA method is periodic, once every 7 days the charger does the equalization.
From my experience I could draw the conclusion that it's something that either needs to be done all the time or at least might take some time to achieve. Most desulfation tests to recover weak batteries seem to run for weeks or even months and results are hardly conclusive. I'm not sure it's a critical thing to be honest, but I can't complain about the life I get from my batteries, either.