These units used to work perfectly well with macrovision on VHS. It would create a new video sync signal that the recorder could understand. They usually only had the video passed into the unit leaving the audio to be passed direct, some of them had other "picture enhancing" options like changing the contrast etc.
It appears that macrovision protection works the same way on DVD, so it will be sending out a corrupt sync signal on the video output which will confuse the majority of video recorders, the same units will work perfectly fine in bypassing this.
It wont work on other types of protection that dont affect the sync signal or any protection that may be put on the audio.
Some DVD players have a hack available to switch off the macrovision chip, or you could get your machine chipped to switch off the protection