Splinter's use of punishments

[Jelly Bean]

Splinter taught Leo that trick because Leo was more eager to learn and showed more aptitude. While Raph was busy arguing with the TV, Mikey was too distracted with his comic books and video games, and Donnie was too mesmerized by his gadgets, Leo was probably begging Splinter to teach him new things.

I agree with this I think Splinter taught Leo that trick because he is more eager over all to learn ninjisu than the others and the others when they are not doing training are off doing other things that interest them. Splinter doesn't play favorites with the turtles.