For Studio 5000, version 24.00, I ran the same installation file and monitored its file and registry activity with SysInternals Process Monitor. I didn't notice anything significant other than the same key being created and set to one.
F:\24.00.00-Studio5000-Web\24.00.00-Studio5000\Tools\Source Protection Tool\RS5KSrcPtc.exe
This time, I noticed that the file's timestamp was 6/23/2005. This tells me they haven't changed their method for enabling this feature in a long time.
The effect on the "Tools - Security" menu was immediate, no application restart was required. Setting the entry to zero also had an immediate effect.