This is the Castle Creations Castle 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 RX disrupts all their 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 an RX, the signal line must be disconnected from the RX.
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 RX to the Castle Link. Easy as that.