It's called compression. The end and exhaust of the rocket system needs to create a higher airstream pressure which can be achieved by a smaller end through which the exhaust gas has to stream.
In a high end version that shoe like upgrade could be used as a thrust vectoring system.
You basically upgrade the rocket with a jet plane part.
Do you understand how rich I would be working for the Empire and not being the Clown of the Rebellion?
Fucking Hell, I'd be one kind of a rich motherfucker!!!!
