All,
Chris suggests that, " You should discuss all this with an experienced A320 base-training captain, as well as seeing what Airbus and your companies say in your Flight Training Manuals."
Very good advice!
And the answer is, from all three sources ... Say clearly "I have control", Push and hold the the Red Button, apply input as required.
The problem with putting in a brief input of any kind without the above procedure is that the pilot flying may choose that same moment to reverse the input that has so alarmed you, and now you are both pushing, pulling or whatever. Oops !!
When completeing Initial Operating Experience Training with new Captains, or Captains new to the Airbus, part of the discussion always revolved around this sort of situation. My advice was always the same ... If you are going to intervene do it early and do it in accordance with the above procedure. And never be afraid to tell the pilot flying to go-around if you do not like what is happening.
best regards,
Bruce Waddington