It would depend what mechanical issues.
Generally throwing a heavy ball is going to get rid of extra, wasted motions. Long arm actions tend to shorten up ect. Same concept as putting weight on a bar and lifting it....in order to lift heavy weight movements have to be efficient.
The other concept that is the same with heavy weights and heavy throws is learning how to do them and working your way up. I would never put 200 lbs on bar day 1 and ask a guy to lift it to fix his issues with a lift. Be careful going out and throwing heavy balls. Randomly doing things like that can be injurious and will (generally) not get the results you want. There needs to be a process in place.
2 lbs usually would be considered a weighted ball.
4 lbs to 10 lbs or more would be considered a medicine ball. Medicine balls throws, things like scoop tosses, rotational throws, overhead slams, reverse overhead throws ect....these are usually used to build explosive power and movement.