Could anyone explain the behavior of DNS slave?

I am testing DNS server, for internal purpose, and after configuring master slave everything works fine, but I am willing to understand the mechanism behind,

Lets say,  

dnsmaster.net and dnsslave.net [my 2 DNS]

However, I wanted to make dnsslave to be available to serve query if dnsmaster.net is down. [well it works until now]

Having said that, I want to update records on dnsslave.net [while dnsmaster.net is down].  

I can resolve things from DNS slave but turning off master and updating records, but not until I restart dnsslave.net.

So, is this something caused by, expiry time on soa?

```
                            43200      ; refresh (12 hours)
            900        ; retry (15 minutes)
            1209600    ; expire (2 weeks)
            300        ; minimum (5 minutes)

```

EDIT: I know what is the purpose of DNS master and slave, I am talking while dnsmaster is down, slave is only in use and I need to update `A` record. [at this condition DNS doesn't work with reload it only works on restart, so anyone with better knowledge and experience please share your view.]