Gez Lemon and Steve Faulkner have gone into more detail on using the virtual buffer in JAWS to enable Ajax interactions.

Popular screen readers use a virtual buffer to allow users to interact with web content, whereby the virtual buffer provides a mechanism for screen reader users to interact with web content. This article uncovers undocumented behaviour in JAWS 7.1 and later, which allows web developers to build Ajax applications that update the virtual buffer without any interaction from the user.

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AJAX, Screen+Reader, Virtual+Buffer, Accessibility