Starting a Wellington

[Wobbley Orange]

Watching the excellent "Target for tonight" courtesy of Talking Pictures from the age where everyone smoked and spoke terribly well. As the Wellingtons start up I saw a ground crew blocking the engine exhaust with, I presume, a rolled up engine cover.
My question is, was this a cold start technique or was there a different reason for doing this?
It was good to see that F for Freddie made it home and that there was eggs and bacon for breakfast!