Cocking the gun tactical motif (preparing a discovered attack)

In Dutch, the two attacking pieces involved in a discovered attack (or the two defending ones in a pin, which is basically the flip side of a discovered attack) are called 'kopstuk' and 'staartstuk', literally translated 'head piece' (the black bishop) and 'tail piece' (the black rook). Preparing a discovered attack like this is called 'staartstuk plaatsen', literally translated 'putting the tail piece in place'. I think it has been coined in the official Dutch method for teaching chess which has been used widely since the 1990's, so many younger and middle-aged players here know the term.

I'm not sure if there's even a well-established (let alone official) English terminology for the 'tail piece'. Wikipedia doesn't mention it. Perhaps 'hidden piece' makes sense? Then you'd be 'putting the hidden piece in place', but as you can see, this is far from being official. I like your own phrase better.