the visportal material on your nodraw brush creates working visportals whereever it can split one sector into two
if there is one tiny leak between those two sectors it will be just one sector and the visportal won't be created (but no evil side effects besides that)
working visportals split up surfaces (so that no surface can be in 2 different sectors at the same time)
"r_showportals 1" does not show the real outline of the created (clipped) visportals. it just shows the outline of the brush face in the editor.
from my experience (but not approved by id) one visportal brush stretched through several different rooms can create several independant visportals. That might have evil side effects and should only be tried if you have a good reason.
imagine this simple hallway
this is the clean way (not really the optimal way, just for show)
this has exactly the same result as the previous pic
the half intersecting visportal does nothing (no good, no bad)
for completeness this is believed to be the optimal way
