Part of the rules seem to have been omitted. There's no reference to the "stacking" part, which is not a real problem if you've played one of the other stacking variants, but is a problem for someone who hasn't. I think this part of the "Stacking Crossdowns" rules should be added in the Play section:

Tiles may be played on the board in blank squares or on squares already occupied by other letter tiles, in which case the tile being played is placed on top of the tile already on the board, effectively replacing it. A tile cannot be replaced for another tile with the same letter on it. At least one tile must be played to an empty square before a move will be considered valid.