Create triggers on slave only in mysql replication with a slave with different schema
@Mati when your Master/Slave is row based replication the replication thread  replicate a copy of the row into the slave instead of ruining the "insert into xx" statement into the slave.

therefore it'll never run your trigger.

however in your case i suggest you the following solution:
1- create synchronized field on the master with default value 0, and never call populate this field during an insert statement from the master
2- create a trigger on the master for each update to set synchronized to 2 when an update occur
3- remove the trigger from the slave

because the "synchronized" field already exists your replication will fail when you create the field on the master at step #1 above. you can tell your slave to skip temporary the error code related to field already exists below:

slave-skip-errors = xxx (xxx is the error code )