IIS7: IServer::AddFragmentToCache. How it is supposed to be used?

I would like to constuct a test response containing HTTP_DATA_CHUNK of
HttpDataChunkFromFragmentCache type. For that I have tried to use
IServer::AddFragmentToCache call. Could someone tell me how to use it?
I have tried to store IServer ptr received in the RegisterModule call. After
that I call AddFragmentToCache inside the PreExecuteHandler notification. I
was able to get nothing except E_INVALIDARG (for fragment names like
"test123") or something like "File not found" for names like
"http://+:80/something" (syntax was taken from HttpAddFragmentToCache
description).