This is the Castle Creations Link Quick Connect.
Experienced modelers go to great pains to tie down their servo and throttle leads, and some find unplugging a Castle controller from the receiver disrupts all their hard work. Many have asked for something like a Y connector that gives an open lead for easy Castle linking, but a traditional Y harness doesn't work when the other branch of the Y is connected to a receiver, the signal line must be disconnected from the receiver.
The Castle Quick Connect does just that; it detects the presence of a Castle Link adapter on the free lead and switches the controller's signal line from the receiver to the Castle Link. it's as simple as that.
Speed Control Accessories: Programmers and Accessories</span