I will discuss the start and the end points in term of the sequence of some important events that a request (and a response) might pass through. See ,  and  for the HTTP pipeline in details.
The following diagram lists some important events in the HTTP pipeline. Most are HttpApplication events.
With the asynchronous PreRequestHandler execution approach, both the start point and the end point are at the stage of the PreRequestHandlerExecute event. The PreRequestHandlerExecute event occurs just before the HttpHandler for the request is called.
With the asynchronous page approach, the start point is at the time when an asynchronous handler is created. This happens after the PreRequestHandlerExecute event and before the Page_Load event. The response data from the Web service call will be available on the Page_Load event handler. The end point is after the Page_Load event (actually after the ProcessRequest call) and before the PostRequestHandlerExecute event.