Krystian's Square is an idea by Krystian Wilisowski. It is arguably the second easiest scramblable twisty puzzle, the easiest one being a 2x1x1, see also this classification by TheMaoiSha. The square can be slid sideways in two ways, enabling a 180-degrees rotation. Krystian's Square is a flat variation to Krystian's Cube.
Watch the YouTube video.
Read at the Shapeways Forum.
Read more at the Twisty Puzzles Forum.
Please order a 3D-printed do-it-yourself puzzle kit from Shapeways at this page (check with Oskar about screws and stickers), or contact Oskar directly if you are interested in obtaining a fully colored, stickered and assembled sample of this puzzle.