Multiple backup in upstream

Question

I want's to understand how multiple backups works

For example, we have 5 instances in upstream

upstream backend {
  server instance-1;
  server instance-2 backup;
  server instance-3 backup;
  server instance-4 backup;
  server instance-5 backup;
}

If instance-1 is down, what backup instance will be accept connetction?

Whey all with balancing, or first in list of backups?

I needs to send all connections to one instance

For example:

• if instance-1 is down, all connection should be sent to instance-2 if
• instance-1 and instance-2 is down, all connections should be sent to instance-3
• etc

How can i realise it?