Scuba and freediver here. This is shallow water blackout, or cerebral hypoxia. It's a protective mechanism, and prevents the person from taking in water. This is why it's crucial for divers to always have a buddy, because there's really no way to tell if it's going to happen. This 'breathe breathe breathe' prompt is important for all surfacing freedivers, as it enforces a CONSCIOUS breath. Smacking the face also helps to wake the diver up - and can be an excellent way to calm down a panic or distressed diver too - the 'snap out of it' response. The rescue breath also signals to the brain that it's safe to breathe again. The protocol at the end (OK symbol and 'I'm OK') is standard (and usually also includes nose clip removal). While it looks scary, it's a really clever physiological response. Without supervision it obviously leads to death. A really important reminder to always supervise young people and divers - free and scuba.
Edit for clarity: shallow water blackout is dangerous and can lead to death. In this case the diver is supported by an excellent team, and my comment about being a 'protective mechanism' is more in relation to the fact it takes some time for this diver to take a breath.
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