The short answer is air infusion or air injection.
An auto-steam wand modification replaces the manual steam handle with a button. Auto-steam requires no input from the barista to texture the milk, therefore, the barista can simply place the wand in the pitcher of milk and the auto-steam wand will do the rest. In most cases, an auto steam wand uses air infusion to create microfoam. An auto steam wand is the quickest method for creating microfoam and provides precise control over the timing and depth of air injection.
Alternative methods for producing microfoam include an expertly trained barista, whisking, shaking, and hand pumps.