Basically when they hit a button it broadcasts S or one of the other action moves. They set this up using their multiboxing software to do so.
If you have IWT turned on for all your actions that's not going to help you as it's going to repetitively spam the IWT button, which is why people will typically bind IWT, use it a handful of times, and broadcast their move button to keep under control. Also keep in mind IWT works with Click to Move as well. So you might want to look into playing around with that turned on. That's what actually leads to runaways.

Back when I had a DK team in WotLK I just set everything to IWT, kept an eye on my slaves and would spam my follow keybind if one started to run away.
I can't speak to which of these solutions is more common. The way I did it was certainly the lazier way, but I also like to do the least amount of modifications in ISBoxer as possible.

Lastly, you might want to post this to the classic forums?