Portals are sometimes helpful. Sometimes they're unreliable, or dangerous. Sometimes, though, they're just weirdly selective. A No-Flow Portal is a strange effect where a portal immersed in whatever medium -- a liquid, like a body of water, or a different kind of atmosphere -- or exposed to conditions which would otherwise affect the other side of the portal, for some reason doesn't let it through even when the portal is working. The Hero can swim into the portal on one side and walk out of it the other side, yet the body of water he swam in remains as stolidly fixed as if it were up against glass. The hero could even cause an explosion next to a portal, and the people on the other side won't even feel a breeze.