Lighthouse uses Emulation.setCPUThrottlingRate command in the Chrome DevTools Protocol:
https://chromedevtools.github.io/devtools-protocol/tot/Emulation#method-setCPUThrottlingRate
You can monitor the protocol this way:
https://umaar.com/dev-tips/166-protocol-monitor/
You'll see this command in the protocol log when you switch with the throttling setting in the performance panel.
If you're asking how to be sure if it works - here is the implementation from Chromium source code:
// This class is used to slow down the main thread for // inspector "cpu throttling". It does it by spawning an // additional thread which frequently interrupts main thread // and sleeps.
Hope this helps.
