A rook attacks along its row and its column, as far as the first piece it meets. In this drill you tap every black rook that could take the white king right now. Find them all and the next board appears.
Get five boards in a row right for a silver star and ten in a row for gold. A wrong tap shows you the answer and starts your count again. On medium and hard, some rooks sit behind others where they cannot reach you.
Tip: look along the four lines out from your king, one at a time. The first rook on each line is the one attacking you.