I think it was the ANC and its activists organizing the coalition of other countries to sanction and isolate the government that ultimately caused it to yield power, which is the necessary condition for any revolution- it requires allies to be in place to support it for when it succeeds. On the ground, you only really need a few dozen people to seize some buildings and bank accounts, it's coordinating the external trade links to keep everyone paid and in their jobs while the top of the regime changes to new hands that's difficult. The opsec for that ground force just has to get most of them to their X day, where they're going to take casulties anyway.
In the case of SA, it seemed like a matter of convincing other countries to do nothing, by persuading the world the govt were just racist villains, and convincing the National Party in government that nobody would intervene to save them if there were a civil revolt. That part was organized in plain view. Opsec is interesting and mysterious, but often less important than the stories we tell about it afterwards.