Connections used to work like this: there were "on left" and "on right" portals. When portals were connected, the combination of their on-left flags and up-vector would uniquely determine which of the two possible portal-matching transformations (mirroring + rotation or just rotation) got used.
With my update, there are only portals with an up direction. Connections are responsible for indicating whether portals are connected with matching up-vectors (and mirroring) or opposite up-vectors (and no mirroring).
Why is this an improvement? Well, portals are stored with their levels, so are fixed for all worlds; connections are stored with worlds (really, connections define worlds). So with this update, world generation has effectively twice the number of possible connections to choose from. This allows for more varied worlds and more replayability.