Strange Effects in 3D Space

[Kitsu]

Actually, yeah, I can see this being a problem in the sonic example, because he's not talking about plain platforms like in Brawl, but rather, solid objects that are in the background. He wants to know if the character is actually standing on those, too (since they're actually on a different plane). This gets especially weird when the player is constantly moving between the two planes; how does the game know where to place the player on the Z axis (in and out of the screen)?

And really, I don't know. The simplest idea is like that vid Rokkan posted.