Fundamentally, Scrabble Overturn is a scoring variant on Scrabble (q.v.)
The board is a simple grid, into which one places "letter cylinders" which depict a given letter in four colors. If a player employs existing cylinders in making a word, those cylinders are rotated to show the active player's color.
Score is totaled only at the end of the game; and only words whose cylinders are entirely in one color count.