Miguel, I'm assuming you're talking about how boxers hit the speed bag?
I've heard it's more of a shoulder conditioning exercise than anything directly related to punching technique,...
...but I don't really see the point, myself.
If so, I'd also be of the opinion that it's a waste of time. There are much better ways to condition the shoulders, and IMO it does nothing useful for one's punching.
I don't consider speed-bag training very useful.
In isolation, perhaps. Combined with other bag work (double-ended bag, heavy bag) IMO it's very good indeed in developing speed and coordination, PROVIDED
you're punching in a rational manner.