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It just keeps track of the position of the corners, and follows them through the moves it does, to make sure that each new move is possible (doesn't split a corner).
Every position in the square one can be reached in 30 moves. But not with an equal probability (many position have more than one solution in less than 30 moves, some have many of them !).
To avoid this, the scrambler use 40 moves instead of 30. More than 40 will becaome very anoying to scramble.
A better version of the scrambler could be selected in the futur, selecting position ramdomly (like on 2x2x2 or 3x3x3), but this isn't the case yet.