Sorry, I tried to google the quote but that was as close as I could come.

I've never fully understood the leash that Vader is on. Without ever getting into it, the OT makes it seem that there is some kind of magical or legal binding on Vader to keep him from betraying Palpatine. This line in particular almost makes it sound like an explanation would be forthcoming at some point in time... I've heard they were working on a Prequel trilogy... I'm sure a totally satisfactory explanation will be given therein. :)

Palpatine implies in this scene that if Luke used the Dark Side to destroy Vader, that Luke's will would be magically sapped and that he would be his new apprentice. In my opinion, Luke does tap into the Dark Side to defeat Vader, but stops short of killing him. It doesn't seem to affect Luke in any immediate kind of way.

It just makes me wonder what Lucas/Kasdan meant by that line. I'm fine with the assumption that they were, at this point in time, just putting in pretty words without a lot of meaning... but I'm trying to give the benefit of the doubt.