You want a slave name server to initiate a zone transfer immediately.
Use the command rndc refresh domain-name-of-zone (for BIND 9) or ndc reload domain-name-of-zone (for BIND 8). For example:
# rndc refresh bar.example
Note that neither command will cause a zone transfer if the master name server has an equal or lower serial number for the zone: the slave will check the serial number, see that its copy of the zone is current and go back to waiting for the next NOTIFY message or for the refresh timer to pop. If you really need to force a zone transfer to a slave, you'll have to delete the backup zone data file and restart -- not reload -- the name server.
Refreshing or reloading individual zones, as shown above, was introduced in BIND 8.2.1 and again in 9.1.0. With older versions of BIND, just use rndc refresh or ndc reload, as appropriate. A full reload takes some time on a name server authoritative for lots of zones, since the name server checks all zone data files to see which have changed.
If you're refreshing a zone that exists in multiple views on a BIND 9 name server, specify the view with rndc refresh domain-name-of-zone class view. For example:
# rndc refresh bar.example in external
Unfortunately, you can't leave out the class, even though your slave name server probably doesn't serve any non-Internet class zones.
5.8.4 See Also
"Controlling the Name Server" in Chapter 7 of DNS and BIND.
BIND Name Server Configuration
BIND Name Server Operations
Delegation and Registration
Interoperability and Upgrading
Resolvers and Programming
Logging and Troubleshooting