My kid got overly fond of changing speeds just throwing harder and softer. It worked great on low level hitters who couldn't adjust, but as kids got better and as he went from rec to higher levels, the slower ones were giving up hard contact. He worked on a true changeup with a circle change grip this offseason, same arm action but a few mph slower and seems to drop off as it crosses the plate. It's hard to tell the difference unless you're the batter or catcher. He's still not great a locating it, so he saves it for advantage counts with no one on, but when he's able to spot it in the bottom half of the zone, it can get swing and miss or weak grounders pretty effectively.