Uhh...Didn't I tell you about them on IRC? Or was that Tapika? ...
Anyways, here goes:
Tag this to an Exit Sector to exit to a custom level, overriding the one set in the map header. The map number you go to is indicated by the front sector's floor. Additionally, if the control linedef's bitset is set to disallow climbing (with the no climb attribute, whose value is 64), skip the score tally screen when switching to the new map.
If the control linedef has the block monsters flag set (value 2), this effect does something super complicated and fun, going to a different level depending on whether the player has all emeralds or not. If the player has eight emeralds (the eight emerald isn't implemented yet, so this is currently impossible), linedef length will be used for the next map. If the player has seven emeralds, the linedef's front sector's ceiling height will be used. Otherwise, go to the map number indicated by the linedef's front sector's floor. That's only if you set the block monsters flag.
Stolen from STARS *cough*