Every coach has different criteria and the details are kept secret. But big plays are always a part of it. I’m sure every TD gets a leaf. Every forced fumble or interception, every sack, every long run or completed pass (maybe 15 or 20 yds or more? I don’t know). They grade each player’s performance after each game so they’re probably also tied to their grade. I know Tressel gave out leaves to the whole team for some things. Everyone probably got one for every win including scout team players. I think I recall hearing about nobody getting a leaf after some particularly bad loss. Can’t recall when that was. (Maybe under Meyer?)
I heard Buckeye fans commenting that helmets at the start of the postseason weren’t as full of stickers as usual and there was speculation that they were being awarded more sparingly to allow for a longer slate of games but that seems kind of fanciful. I didn’t pay that much attention to it.