Fast replication

[@4OS]

Hi,

I'm trying to get replication to work with a 2 machines setup.

So far full replication is happening at the specified interval (24hrs) 
without problem but fast sync is not happening.
The fast sync counter is always equal to full sync unless i exec dsync 
manually.

doveadm replicator status '*'
username priority fast sync full sync success sync failed
johndoe at oricom.dmz none     00:06:33  00:06:33  00:21:33     -
johndoe2 at oricom.dmz none     00:06:34  00:06:34  00:06:34     -
johndoe3 at oricom.dmz none     00:06:34  00:06:34  00:06:34     -

The fast replication part is not clear and i cannot find much about it.
There is nothing related with dovecot -a and the documentation does not 
say much about it.

How the fast replication is triggered? Manually or it is supposed to be 
automatic through some type of config.

Some light over this would help me understand the way it work and how to 
get it setup right.

yves