Birdie: Check this out. You ain't goin to college or any fuckin where if the Bird Man don't win. You might be on the other team, but you playin for ME. Remember that.
Shep: You hear that Nutso? The boy here says he owes me. he owes ME. Are we even? ARE WE even? You don't owe me nothin. You owe yourself and ones who cared to get you here. Believe me. I know.
Kyle-Lee: Why are you doin this man? It's just a game.
Shep: Not to me.
Bugaloo: Man, you look like Daffy Duck with his beak shot off.
Answer: There is no in-film explanation for this nonsensical scene. It's a dilapidated hoop on a roof, so maybe we can assume there used to be safety netting there at some point, but it has since been removed or destroyed. The real-world answer is the filmmakers wanted the death to be a freak accident. It also had to be something Shep would be legally blamed for (the police thought Shep threw Nutso from the roof) and something that would make Shep feel immense guilt (he bet Nutso couldn't jump higher than he could). And of course, to make it topical to the plot of the film, it also had to occur during a basketball game. This is apparently the best they could come up with, and we are left with a classically ridiculous death scene that makes no sense at all.
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