With Asynchronous->True, the WolframAlpha function sets the flag in the API. An initial block of content is retrieved from the Wolfram|Alpha servers, which may contain placeholders. The placeholders are filled by subsequent, parallel calls to Import. The WolframAlpha function returns once all of the calls to Import have been resolved.
WolframAlpha["query", "FullOutput", Asynchronous->All] retrieves the initial block of content identically to Asynchronous->True. The function then immediately returns, displaying Dynamic content that populates over time with subsequent calls to Import.