In this article's video, Dan Barbera wonders when the vapor chamber kicks in. It's always operating, since it's a passive component, and doesn't contain any of its own electronics, so it's nearly always redistributing and dissipating heat while the iPhone is powered up, starting at relatively low internal temperatures since the lower-than-atmospheric pressure inside a vapor chamber allows the water inside to not wait until it reaches 100C to evaporate, but rather starts doing this at lower temperatures like 30-45C (86F-113F), which some of the parts inside smartphones reach pretty soon after you power them up and start using them.