LayoutPostAsync turns on and off asynchronous IO for data posted to forms. This allows you to read data multiple times without fear of locking up the socket. This means that your header and footer can now have access to the data POST'ed from a form.
Known ProblemsThis has a cost to it, so you are better off not enabling it if you don't need it. Systems which have tmp file systems have little impact from using it. See the LayoutPostAsyncCache directive for how to move the cache if you need to.