BeforeNavigate2 event and PostData

I'm capturing the BeforeNavigate2 event of a WebBrowser control and I tried to edit the PostData parameter from here, but it won't send my updated data (just sends the original PostData).

Code:

void BeforeNavigate2(IDispatch *pDisp, VARIANT * url,VARIANT * Flags,VARIANT * TargetFrameName,
 VARIANT * PostData,VARIANT * Headers,  VARIANT_BOOL * Cancel) 
{
		SAFEARRAY* psa = (SAFEARRAY*)SendMessage(msgForward, WM_USER+0xdead,
			(WPARAM)0, (LPARAM)PostData->pvarVal->parray); //retrieves modified post data into psa
		SafeArrayCopyData(psa, PostData->pvarVal->parray); //applies the modified data
}

I've made sure it all works in debugger, isn't this the place where modifications of PostData before request can be made?

According to the manual page - did you read it, or just assume what you were trying would work, the only thing this function can do is decide whether or not to allow the transfer.
It does that by setting *Cancel = TRUE;

Oh, and it says some other stuff as well.

I didn't read it all, however, it would make much more sense if I could edit the variables in this function, instead of cancelling the current request and making another one, wouldn't it?

Thanks for enlightening me on this.