Currently, when you hold up the iPhone X and Face ID fails, you either have to lower it and raise it again, or press the side button to put the iPhone to sleep and wake it up through another side button press or tapping the display. You can also tap "Cancel" after the failure then swipe up to start over. All of these methods will reactivate a Face ID scan.
As discovered by Redditor shakil-ali, Apple has provided a slightly simpler way to rescan your face in the iOS 12 beta: now when the biometric security feature fails multiple times you can swipe up from the bottom of the iPhone X to get Face ID to scan again, without having to tap or press another button first. This should help to alleviate a small but sometimes frustrating part of unlocking the iPhone X with Face ID.
In regards to other hidden features of iOS 12, we have collected a bunch in a tidbits roundup posted yesterday. These include setting up an "alternate appearance" for Face ID, a new QR code scanner in Control Center, a revamped Battery Usage section in settings, and more.
As a whole, Apple announced iOS 12 with a focus on user privacy and digital health, and when the update launches in the fall iPhone and iPad owners will be able to keep track of everything they do on their devices with features like "Screen Time" in Settings.