If this suggestion or Chilly Willy's is able to make Doom run at 320x200 and 20-30fps fairly well then they would be likely candidates for what the trick is.
I don't think any of them is a candidate.
Chilly Willy's trick sure isn't since he's talking about cutting vertical resolution in half (display ~100 lines instead of ~200). So this is not about running Doom in 320x200.
Using GPU RAM as a cache? I'm not sure. It is is mentionned in the Jag Underground Dox from 1997. So it was most likely already known when Battlesphere was in developpement.
Well, it would be 320x200, but every other line would be a repeat of the previous.
Between drawing half the data and freeing half the bandwidth to other things, it's clearly not the blitter trick because you'd get BETTER than what they claimed for Doom.
As to using GPU ram for a cache, I don't see that as possible on a game like Doom where even the floor/ceiling textures are 4K to begin with. Forget about wall textures. So that's not a candidate for the Blitter Trick either.
Zerosquare's suggestion is a likely candidate.
No, that can't be it either or they wouldn't claim it would speed up Doom. Drawing every other pixel isn't going to help Doom any.
Anyway I'll try for time to time to torture the Blitter.
If I find something, I'll tell you all of course. I mean I'll say: "Behold my superior intelligence!!! I found the Blitter Trick!!! But I won't tell you what it's about!!! Your puny littlle human brains would never understand!!!"
EDIT: One thing I think maybe worth trying when I get around to it... Doom is primarily rendered as columns. That's not very fast on many systems, so turn it around - render Doom as ROWS at a 90 degree angle, then use the OP to flip it horizontal during display. Rendering rows with the blitter in Doom should be faster than rendering columns. However, that would make the OP use more bandwidth, so I'm not sure if the tradeoff is worth it.
Edited by Chilly Willy, Fri May 18, 2012 5:46 PM.