It's how OpenGL works. Basically, it's the lack of depth clipping or a depth buffer.
In other words, the renderer draws the platform as if it would be at that position. Then, the next object is drawn over the last, where it would be as if nothing else was there. Normally, however, the renderer will hide things that are blocked in 3-dimensional space, but this can be disabled. In Sonic, however, the images are not rendered realtime; they're just sprites.
In other words, the renderer draws the platform as if it would be at that position. Then, the next object is drawn over the last, where it would be as if nothing else was there. Normally, however, the renderer will hide things that are blocked in 3-dimensional space, but this can be disabled. In Sonic, however, the images are not rendered realtime; they're just sprites.