Check out this fun youth play:
Here is a great example of why umpires have to know their differences. When this play happens, it will have the element of surprise – a killer for umpires. The rest of the play can get jumbled in your brain.
As far as the rules go, this is legal in professional baseball. A lot of youth organizations base their rules on professional, so it is legal in many of them (I think – to be fair, I don’t umpire a lot of youth ball). If it is technically legal in youth leagues, may local leagues bar the practice citing safety issues.
You can also do this in college baseball.
You cannot do this in high school baseball.
Rule 8-4-2 (b) states:
Runners are never required to slide, but if a runner elects to slide, the slide must be legal. (2-32-1, 2) Jumping, hurdling, and leaping are all legal attempts to avoid a fielder as long as the fielder is lying on the ground. Diving over a fielder is illegal.
It is odd that something is allowed in youth but not in high school? Maybe. That does not stop umpires who work multiple levels from being familiar with all the differences.