I never tell my pitchers, or any for that matter "son, you need to go out there and throw your fricken arm off". I am not advocating that they get out there in games and throw as hard as they can. When they do their long toss, bullpens, etc, in "practice" they should work on trying to throw harder at some point. That is why I said "practice". For instance- Near the end of a bullpen, our pitchers will pony it up and throw some of their last few pitches harder than their normal cruising speed. This is what is meant by practicing to throw harder.
I hope that clarifys things.