Boost hit one bar today - MINUS one bar!

Jon has already told me what it might be, but would like a techie viewpoint

Driving along in gear, but with clutch in as I was slowing down for a red light coming up. Heard the exhaust change note slightly, and looked down at the boost guage only to see the guage ZIP down to -1 bar and then back up to 0 again. It might have gone even lower than -1 as that's the lowest it reads (i.e it hit the stop)

Words like actuator / wastegate / vacuum have been batted about. Any clues. Might have to get this laptop screen fitted quicker than I first thought.

once it had this 2 second blip, it was driving totally fine - no limp mode or anything. I made sure of that away from the red light. vvvRRROOOOMMM