How does IIS Health Ping actually work?

I'm developing Native Modules for IIS7 and could find nothing definitive about the working details for the IIS Health Ping. Is only the main thread for w3wp.exe "pinged" and how? Or is every child thread that is handling a URL request also pinged? If only the main thread process is pinged, then after RegisterModule() returns, it's not clear to me how anything I do in the request handler class could impact the Health test and that the heath check would fail to detect a hung request on a child thread.