Holy banana's, Infero that's an elusive reply! ha-ha.
I did not know you were into hammering your drums AFTER they were manufactured (just joking my friend)
The hammering of a metal shell does 3 things, apart from making it prettier
and increasing the price.
1- It makes the shell stiffer, like the articulated folds or ridges on the metal body work for cars.
2- It gives a slightly bigger (developed) surface area on a given size of the shell.
3- It difuses the reflection (directions) of the sound, making it more omni directional. In other words there are more random reflection angles with a longer distance to travel before they hit another part of the drum. (shell and or head)
The overall effect is a touch richer and more sophisticated sound. I like it a lot.
As for a sharp crack, the smooth metal shell reigns.
Cheers and Happy drumming,
Harry