http://docs.mongodb.org/manual/tutorial/configure-replica-set-secondary-sync-target/ indicates that if an initial sync is already in progress then rs.syncFrom will have no effect, nor will it display an error. This command shouldn't silently fail. It should display an error in the shell indicating that it didn't succeed.

The output:

rs.syncFrom("HOST:27017")
{ "syncFromRequested" : "HOST:27017", "ok" : 1 }

doesn't indicate that nothing will happen until the initial sync completes.

The docs for rs.syncFrom http://docs.mongodb.org/manual/reference/method/rs.syncFrom/
and replSetSyncFrom http://docs.mongodb.org/manual/reference/replica-commands/#dbcmd.replSetSyncFrom
could indicate this better. The only mention of this behavior is on the page linked at the beginning of this report.