Here's a short write up published on Boardgamegeek:
Each player commands the usual named pieces in chess: 1 King, 1 Queen, 2 Bishops, 2 Knights, and 2 Rooks. Before the game begins, the player has the opportunity to swap out named pieces for pawns (at the rate of 2 pawns per piece exchanged). Each playing piece has it's own special rules (all contained in the Song of Blades and Heroes rulebook, no other expansion is necessary).
The aim of the game is to take out your opponent's King by killing him or making him flee the board. Although chess pieces and board are used, this is not a chess variant - it is Song of Blades and Heroes to use with your chess set. The pieces retain the character of their equivalent pieces in chess, within the SBH setting.
The idea of Song of Rooks and Pawns is to offer a gateway introduction to the Song of Blades and Heroes system.