I would like to know the answer to this too. According to the embedsMany documentation:
Treat embedsMany as an actual relation, no different from hasMany, for example. This means that you cannot just POST the full object with embedded/nested data to create everything all at once. So using the example above to add a Customer and multiple email addresses would require two POST operations, one for the Customer record and one for the multiple email address data.
Give this, I don't actually know what the difference between embedsMany and hasMany is.
-Josh