Board vision training: is the square light or dark? On the board or by coordinates only.
A player who knows at once that e4 is light and d4 is dark sees bishop moves faster, knows where a knight can go and calculates more easily. It is the basis of blindfold chess.
The trainer marks a square on the board or shows only its name, and you answer: light or dark. On a one-color board or without a board you have to recall the color, not see it.
a1 is dark. If the file number (a = 1, b = 2 and so on) and the rank are both even or both odd, the square is dark, otherwise light. e4: 5 and 4, different, so it is light.
To recall the color instead of seeing it. It prepares you for calculation and blindfold play.
Yes, the trainer is free and works without sign-up.