The idea is a simple one: If you where to take a standard chess board and design a set of pieces off the 40k model lines which models would be what? Heres an example...
40k Tyranids
King - Hive Tyrant
Queen - Broodlord
2 Bishops - Zoanthropes
2 Knights - Lictors
2 Rooks - Carnifexes
8 Pawns - Genestealers