[frodo3212]

This article mentions pop voice specifically, but doesn't delve into the actual physiological phenomena associated with head voice/falsetto, in part because of current limitations at understanding how we make sound. Male head voice and falsetto are related to crico-thyroid (antagonistic muscles to vocal folds) dominant phonation, as opposed to thyro-arytenoid (vocal fold) dominant phonation. The differences from there can be attributed to a number of additional factors, like subglottic air-pressure or pharyngeal shape. If you're interested in further reading or research on the subject, the writings of Richard Miller or Ingo Titze would be great places to explore.