This is a mobile form of the portal used by the Dark Eldar to link together places via the webway. It may be activated by the model carrying it in the shooting phase, instead of moving or shooting that turn. Place a spare Blast marker in base contact with the model when it activates the portal. From then on units entering play as reserves may be placed on the portal template instead of entering on the table edge. There is no risk of scattering or being killed when travelling in this way, and the portal cannot be destroyed. Once activated the model that was carrying the portal may move away, and doesn't have to remain in contact with it. If you have a webway portal in your army, you may keep units in reserve, even if you are not normally allowed to do so in the mission being played. If you do this then the troops may only enter through the webway, and if it has not been opened on the turn they become available to enter play, they must be held back until it is.
Errata — Chapter Approved: Codex: Dark Eldar Q&A (v2.0): Treat the edge of the portal marker as a small piece of your own table edge — models entering from reserve move onto the table from the portal marker, measuring from its edge exactly as they would entering from a normal table edge. The normal restriction on coming within 1" of an enemy model still applies (except during an assault), so if enemy models surround the portal it cannot be used to enter the table. Units may not partially enter play using the portal, nor may they charge through it the same turn they arrive. In short: defend the portal until you've used it, and don't leave it abandoned in the middle of the enemy.