45-degree demo start

This might be a really dumb question, but here goes:

I've seen some multi-level Doom/Doom II demos where the player has managed to be turned 45 degrees or so, even before the screen melt finishes. I figured that was like an SR-40/SR-50 thing, but how is it accomplished, before the first tic of the level even?

I feel like this should be very obvious, but I can't figure it out.

http://www.doomworld.com/vb/source-ports/29654-prboom-turn-snapping-bug/
http://www.doomworld.com/vb/doom-speed-demos/43359-recording-from-the-first-tic/