The index API allows one to set the threading model the operation will be performed when the actual execution of the API is performed on the same node (the API is executed on a shard that is allocated on the same server).

The options are to execute the operation on a different thread, or to execute it on the calling thread (note that the API is still asynchronous). By default, operationThreaded is set to true which means the operation is executed on a different thread.