The muscular arm and raised hammer represent Vulcan, Roman mythological god of fire and metalworking, and was once the symbol of the Vulcan Spice Mill in Brooklyn, N.Y. James A. Church, owner of the firm, took the symbol with him when he shut down the mill in 1867 and went into business with his father, a baking soda manufacturuer. Shortly afterwards, the "power" behind Vulcan's arm and hammer became the trademark of Church's baking soda, which had the "force" to make bread dough rise.
My grandfather told me this, apparently he used to work in a baking soda factory.