Legacy Floor Over Floor

How exactly do I do the floor-over-floor effect in Legacy?

Make a sector alone by itself (the control sector) and give one of the linedefs the type# 281. Now tag it to the sector(s) you want the 3d floor to appear in. The floor height and texture of the control sector corresponds to the bottom of the 3d floor and the ceiling with the top. The light level of the control sector is the light level under the 3d floor (the original sector's light is the light level above it). The texture the sizes of the 3d floor uses comes from the linedef 281.

I got loads of Legacy editing tutorials on my editing site. The link in my sig is broken, use this one:

http://www.telusplanet.net/public/daewoo/der/