What it does
Picks whether Shake It On shakes on every interval (always active) or only when the system has been idle for at least one interval (only when idle).
How it works
In always-active mode, the engine shakes every N seconds when conditions are satisfied. In only-when-idle mode, the engine reads the system idle time (via CGEventSource) and only shakes if it exceeds the interval — meaning you weren't using the Mac.
Where to find it
Settings → Activation. Radio group with two options.
When to use it
'Only when idle' is gentler — Shake It On stays out of the way while you're working and only kicks in when you walk away. 'Always active' is what you want if you're trying to outsmart a presence-tracker that requires constant motion.