Dynamically adding a replica node doesn't do anything

Hi to all,
I was playing with ES in order to test dynamic scaling. I have a working cluster with some indices and I want to start replicating one of them. I tried to use curl -XPUT 'master-node:9200/my_index/_settings' -d '{ "index" : { "number_of_replicas" : 1}}' but the other node doesn't start syncing the data.
I tried also with number_of_replicas = 2 but the replica node doesn't download anything but the _state folder for each index.

Am I doing something wrong or is it normal? I tried also to force the flush through my_index/_flush/synced but nothing happens..

There's no such thing as a replica node, ES balances the shards (primary or replica) across all nodes in the cluster.

So there's no way to force ES to start replicating an index in the cluster once I need it?

Yes, you can add a replica to an index if it doesn't exist.

So why in my test cluster the added node didn't start synchronizing the data from the master?

Did the node join the cluster?

Yes, it did. When the node join the cluster it creates all indexes folders in the data directory but they stay empty (except for the _state directory).

Did you change elasticsearch settings? May be your node is a master node only?