How does nbsrv works in this case?

Hello,

Does nbsrv directive its considering backup servers as usable servers once they fire up ?

Basically I want to make the switch to the webservers_backup backend if there are less than two usable servers in the backend webservers. Another option I am thinking its to make this switch only if the regular servers are down ( cache1, cache2 ). Is this possible ?

Example:

acl webservers_dead nbsrv(webservers) lt 2
use webservers_backup if webservers_dead

backend webservers
option forwardfor allbackups
server cache1 192.0.0.1:8000 cookie web1
server cache2 192.0.0.2:8000 cookie web2
server web3 192.0.0.3:80 cookie web3 backup
server web4 192.0.0.4:80 cookie web4 backup

backend webservers_backup
server web5 192.0.0.5:80 cookie web5

Thank you.