Not again!!
O.K., just this once..........
It's the P.T.U: the Power Transfer Unit. It's a device which transfers hydraulic power from one system to another, whilst remaining hydraulically de-coupled. Under certain conditions (like the engine start sequence here) it will operate automatically.
We engineers also use it to power up the main hydraulic systems (yellow and green) via a single electric pump (yellow system).
Hope this helps!