Multiple backup in upstream

Hi!

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?