Spitting a shell towards a wall just before the peak of Yoshi's jump will cause the shell to rebound off the wall and land under his feet. When Yoshi spits a shell he is not able to interact with it for ? frames. This means he needs to be at least 2 blocks from the wall in order to bounce off the shell.
When Yoshi spits the shell, there is a 2-3 frame window for it to land underneath his's feet.NoteWhen Yoshi is close to a wall, there is a 3 frame window. While when Yoshi is further away from a wall there is only a 2 frame window.