Necron portals are the means by which the Necron can move onto the board, transport around the board, and withraw back into the reserves. Ã‚Â Portals are not used by the units bearing them (indeed, not all portals are IN units), but are used by the formations transiting them.
There are a few ways of using the portals.1.)Portal in / Withdraw:
The most basic hit and run. Ã‚Â The formation using the portal begins in the Reserves. Ã‚Â They move out via a porta, ending their movement within 5cm of a second portal. Ã‚Â This is used by assaulting formations. Ã‚Â Should they win, their 5cm consolidation move would allow them to move into the portal, and back into the reserves. Ã‚Â If not all units are within movement range of the second portal, they will be destroyed (as per the rules for a unit that finishes its move outside of cohesion).
If they are further than 5cm away, they would only be able to withdraw via portal if they become broken, and may make their retreate move through a portal if they are able.2.) Move Through:
This use is very easy. Ã‚Â Basically you move your formation into one portal as part of its movement, and then out of another, continuing the formation's move from that point. Ã‚Â It's really not a very efficient way to move most troops. Ã‚Â It was intended as a means by which you could move Wraiths, Destroyers/Heavy Destroyers and Flayed Ones taking an engage action across long distances.
Attached file shows examples, complete with images.