I couldn’t find the information on the talk we had but I did have a browse through a couple of Mosquito books and this is from de Haviland Mosquito, Martin W. Bowman:
The first version of the segmented Youngman frill airbrake was fitted to W4052, the fighter prototype, when the aircraft was at Salisbury Hall. The ‘frill’ could be opened by a bellows and venturi arrangement to provide rapid deceleration during interceptions and was tested in a number of different forms between January – August 1942 but the idea was shelved when it was found that lowering the undercarriage in flight had the same effect.
I thought it might be of interest.