How DocumentSink, DocumentSource ops in TF Syntaxnet actually work?

Question

After going through the code... I could understand the flow as..

In parser_eval.py

sink_documents = tf.placeholder(tf.string)
sink = gen_parser_ops.document_sink(sink_documents,
                                      task_context=task_context,
                                      corpus_name=FLAGS.output)
sess.run(sink, feed_dict={sink_documents: tf_documents})

And going through the gen_parser_ops.py I found

def document_sink(documents, task_context, corpus_name=None, name=None):
  r"""Write documents to documents_path.

  Args:
    documents: A `Tensor` of type `string`. documents to write.
    task_context: A `string`.
    corpus_name: An optional `string`. Defaults to `"documents"`.
    name: A name for the operation (optional).

  Returns:
    The created Operation.
  """
  result = _op_def_lib.apply_op("DocumentSink", documents=documents,
                                task_context=task_context,
                                corpus_name=corpus_name, name=name)
  return result

But I couldn't understand how the below piece of code is saving the documents to a file.

_op_def_lib.apply_op("DocumentSink", documents=documents,
                                task_context=task_context,
                                corpus_name=corpus_name, name=name)

and also it would be a great help if anyone could help me provide with small example on how to use the op_def_lib of TensorFlow.