You're in a grotto that goes on in a fourth direction, w, which you can't see: you see the 3D slice of it you're standing in. Its crystals shine in all four, so light reaches you from places you can't see, and shadows fall from rock that isn't there. Find the crystal.
Turn, and w turns with you: you turn in a plane that leans into w, so turning round swings w into the slice you see, and what you came past slides out of it. Walk where you face, w included, and step up a ledge without thinking. Twist to lean w in sideways.
Then flip: pick a wall, ahead, behind, left or right, or one of the two you can't see, ana and kata, and it becomes your floor. Floors stay flat whichever way you face, because gravity only ever falls along the cave's grid. You need room to flip: about six voxels clear round you, w included.
WASD walk · mouse turns and looks (click to grab it) · Z / C twist · arrows flip ahead, behind, left, right · Q kata, E ana · R restart. On a phone: the stick walks, drag to turn and look, the pad flips and twists.