Unlike an iron, which relies on mechanical force (flattening through the weight and friction of a hot soleplate), a clothes steamer works through heat transfer and immediate moisture input.
As it passes through the fabric, water vapour (between 98 °C and over 140 °C depending on whether it comes from a continuous flow or a pressurised boiler) penetrates to the core of the yarn. Heat combined with water molecules temporarily breaks the hydrogen bonds that hold the fibre in a creased state. Suspended vertically, the garment regains its natural alignment and three-dimensional structure under its own weight, without any abrasive stress.