I wonder if it is like the Interferential stimulation machines?
the power signals interfere with each other so less chance of tingle/zap, stinging. These work deeper into the muscles/tissue too.

When my chiro got his INF stim he asked if they could use me as a guinea pig LOL - the tech/trainer for the stim company was there to train him.
The tech had him start at a fairly high setting instead of starting low and working up to my tolerance level. But they listened to me and turned it down fast.
A few times later I let him turn it so it was really flexing my muscles- it didn't hurt but I ended up with triggerpoints afterward-so lesson learned.
One time it flexed my shoulder muscles so much I thought it was gonna pop out of the socket.
So now we just keep it to low levels so it feels like a wonderful full back massage.
But in my case it's for the TOS and myofascial pain- so I don't know how it would apply to cases of RSD.

any time they turn any of those electric stimulator's up too high you will feel zapping , shock or stinging.

If you do it and you feel it is too high of a setting- demand they turn it down- just because those settings are the norm doesn't mean everyone can tolerate it.

I have a cheap muscle stim {EMS} for home use and it pretty much only works on the surface of the muscles compared to the INF stim my chiro uses.