Quoting myself:
"They're basically like sophisticated buttons. When the player steps in a trigger sector (marked by the appropriate sector type), it looks for a tagged linedef for the trigger. Call the sector that linedef's front belongs to the "command sector." SRB2 then goes around the command sector and "executes" all the linedef commands (100-104 as listed in my effects guide)."
The numbers now go from 100 to 111, but the basic principle still holds. After FD 1.09 is released, I'll definitely make a linedef executor tutorial guide (maybe sooner since this is the third topic asking how linedef executors work).