According to Patently Apple, the front-facing camera takes rapid snapshots that then translate into an action. If a phone is on but the camera can't see a person looking at the display, it'll lock the product. Conversely, if a person picks up the device, the camera will recognize that and unlock it.
Apple also describes the use of sensors in a respective device that can determine whether a device should be locked or unlocked. For example, the respective device could sense someone holding it, gripping it, or moving it in some fashion, and then execute a locking or unlocking function.
It's the face, though, that has proven most appealing to companies. Back in December, in fact, Apple filed for a patent on a technology that would recognize a person's face and use that as the authentication needed to access user profiles or other important information. Earlier this month, Google was awarded a patent on a technology that will identify a person's face and deliver full access to personal information.