Your talking about "Native Resolution" video output mode on the HR20 HD DVR, right?
I think DirecTV is recommending to keep it "off" because it currently makes channel changes very slow. The DVR takes several seconds to determine that channel's video resolution format, and then your TV/monitor has to resynchronize to that video stream as the format changes between 480i, 720p or 1080i (depending on the channel). If you turn off "Native Mode", the HR20 does the video scaling and the TV only sees the fixed resolution being output by the HR20.
Do a Search in the HR20 Q&A Forum here and you'll find much more information about the pros and cons of "Native Mode".
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