My daughter has been going on it since probably slightly under 40 inches (without shoes she was definatly under 40 inches). She's been riding since she was 3. For about 3 to 4 months is was hit or miss on wether she would get on it, depending on what shoes she was wearing, how alert she was (they once tried to measure her when she was half asleep and didn't stand up as tall), and what the CMs said. They'v since changed the height indicator for that ride, but I'm convinced that it used to be highter than 40 inches (even though thats what it was supposed to be) because no other rides she would have a problem with but that one. They have a strap that kids have to use that goes between their legs, and the lap belt goes through it.
Also some Disney hight indicators are a line, and others you measure under (they have a sigh with 2 poles on the sides, or something sticking out). In those they expect you to hit the indicator. My daughter used to duck and walk under because she didn't want to hit her head. Finally I realized the problem, and told her, no you're supposed to hit your head or they won't let you ride. So know she knows that she's got to stand up tall (without tippy toes) and bump her head.