Job update documentations

In the update stanza of documentations there is a health_check parameter that specifies the mechanism in which allocations health is determined.
The default value is “checks” which means that health of the deployment is determined using Consul checks.
The interesting thing is that documentation mentions that check_restart is disabled by default.

My question is how does health check work while it is disabled by default?

Additional question: How does nomad determine job health after first deployment? It looks like update stanza is not functioning at that point. I haven’t been able to understand this by reading documentation.