Adobe has publicly documented the Real-Time Messaging Protocol (RTMP) that is used by Flash for streaming video, but the company has fiercely guarded RTMP content protection measures, making it impossible to create a fully compatible open source RTMP client. SWF Verification is one such security measure.
An RTMP streaming video server that has SWF Verification enabled will terminate connections from clients that fail to supply an authorization key. The purpose of this restriction is to ensure that the content is only accessible to specific SWF files, thus preventing third-party software from downloading the video.
Read more at Ars Technica