What it teaches
Everything the three puzzle games teach, used at once. A child who knows how a bishop moves still has to decide which bishop to move and where, and that decision is the game.
All the rules are here — castling, en passant, promotion, stalemate and the draws — so nothing learned at this board has to be unlearned at a real one.
How it works
Tap a piece and every square it may legally move to lights up: a dot for an empty square and a ring around a piece that can be taken. Tap one to move. Nothing is dragged, because a piece halfway between two squares has nowhere to be.
Three strengths. The weakest looks one move ahead and chooses at random among its better replies — not among all of them, because a computer that plays a genuinely random move is not an easy opponent, it is an incoherent one. The strongest looks three moves ahead, which will beat most eight-year-olds.
Take back undoes your move and the reply together, as many times as you like. A child who cannot take a move back stops trying things.
What to look for
Start at strength 1 and stay there until it is being beaten regularly. Losing every game teaches nothing, and the point of this one is that it can be won.
The bar under the board shows the pieces each side has lost and who is ahead on material. It is the single most useful thing to point at: most games at this age are decided by one side giving pieces away without noticing.