In video games, particularly 2D platform games, there are sometimes some weird platforms and surfaces. The most common variation of them is when you can jump through them from below, yet they are solid when you're above them. In 2D platform games, it can be assumed that your character is not jumping 'through' the platforms, but rather jumping 'in' or 'out' of the 2D plane (ie. the platforms may not be in the same plane; it just looks that way to the player).