Cornstarch does one thing and does it well: it absorbs moisture. That sounds unglamorous until you understand that almost every unpleasant thing that happens to a foot — the smell, the fungus, the blister, the softened skin that tears — begins with the foot being damp inside a shoe for hours.
It also reduces friction. A blister forms where skin rubs repeatedly against sock or shoe, and a dusting of cornstarch lets the two surfaces slide rather than drag. For anyone who walks or stands all day, that is genuine prevention rather than treatment.
The book puts it alongside baking soda, and it is right that they overlap. Cornstarch is the better absorber; baking soda is the better deodoriser. Many people use both.
