If you set light to a small amount of the nitro-cellulose propellant powder used in modern shotgun cartridges (and don’t try this at home!) you will find it burns with a fizz rather than a bang.
In other words, it burns much more slowly than it does when you are shooting – firing a cartridge in a gun.
In order to burn fast enough to propel a shot charge through a barrel in a few milliseconds, the combustion has to take place in conditions of considerable pressure.
This pressure starts to really build up as the shot is forced through the crimp.
An eight-star crimp is a bit more resistant to opening than a six-star, so the more resistance there is, the higher the pressure.
And the higher the pressure, the faster the powder burns.
The number of stars in the crimp are therefore one factor in regulating the performance of the cartridge.