From SMOKtech this is an adjustable air flow controller and platform for your tanks. This threads into any 510 connection.
Generous air divets (four of them) encircle the base. When the adjustable ring is screwed all the way down it completely covers them so air flow can only come from the connector area. With this innovation you can get more air flow by adjusting this ring up. It is recommended to screw all the way up to completely open the air divets, then attach your tank to the platform, then screw back down to close off and restrict flow as necessary.
This device will allow you to use a Locking SmokTech tank on a ProVari also, as normally the Locking tank would seal the air from flowing through the tank.
Many people, like us here at Zeus E-Juice, use them to keep the threads of their MOD from getting damaged. When you change devices often, or drop your MOD, the carto/tank/atty can actually strip your threads if it gets yanked out. With this, if the threads get stripped, you only lost a few dollars, instead of your whole MOD.