It is an adapter/barrier vehicle, as the HST Mk.3s have a fixed Buckeye on a sprung drawbar, called an 'Alliance coupler'.
GBRf 66s only have traditional Drawhooks and Shackles, as you can see on the front of the 'Gred' in the above picture, and the Mk2s have Drophead buckeye couplers, and so one end can be attached to the HST Mk.3s with the Buckeye attacment raised, and the other end can attach to the Locomotive by dropping the buckeye doan and using the conventional drawhook & Shackle.
The entire formation is air braked, as the air connections on the main res. and brake pipe hoses are all universal, so brake force is not required in this instance.
Hope this answers all your questions.