diff --git a/niworkflows/engine/plugin.py b/niworkflows/engine/plugin.py
index 8c074e9cbe8..14c2547f0c3 100644
--- a/niworkflows/engine/plugin.py
+++ b/niworkflows/engine/plugin.py
@@ -401,6 +401,16 @@ def __init__(self, pool=None, plugin_args=None):
         """
         Initialize the plugin.
 
+        If passed a nipreps-style configuration object in `plugin_args["app_config"]`,
+        the following fields must be present:
+
+        app_config.environment.total_memory : :obj:`float`
+            Memory available to the workflow in gigabytes.
+        app_config._process_initializer : :obj:`callable`
+            A function that accepts a file path and returns None, to be run in each worker.
+        app_config.file_path : :obj:`str`
+            The path to a file that will be passed to the initializer.
+
         Arguments
         ---------
         pool : :obj:`~concurrent.futures.Executor`
diff --git a/niworkflows/engine/tests/test_plugin.py b/niworkflows/engine/tests/test_plugin.py
index e17ecd3f903..6956ba19c6f 100644
--- a/niworkflows/engine/tests/test_plugin.py
+++ b/niworkflows/engine/tests/test_plugin.py
@@ -72,28 +72,28 @@ def test_plugin_defaults(workflow, caplog):
 def test_plugin_args_noconfig(workflow, caplog):
     """Test the plugin works with typical nipype arguments."""
     caplog.set_level(logging.CRITICAL, logger="nipype.workflow")
-    workflow.run(
-        plugin=MultiProcPlugin(),
-        plugin_args={"n_procs": 2, "memory_gb": 0.1},
-    )
+    workflow.run(plugin=MultiProcPlugin(plugin_args={"n_procs": 2, "memory_gb": 0.1}))
+
 
+def touch_file(file_path: str) -> None:
+    """Module-level functions play more nicely with multiprocessing."""
+    with open(file_path, "w") as f:
+        f.write("flag")
 
-def test_plugin_app_config(workflow, caplog, capsys):
+
+def test_plugin_app_config(tmp_path, workflow, caplog):
     """Test the plugin works with a nipreps-style configuration."""
 
-    def init_print():
-        print("Custom init")
+    init_flag = tmp_path / "init_flag.txt"
 
     app_config = SimpleNamespace(
-        environment=SimpleNamespace(total_memory_gb=1),
-        _process_initializer=init_print(),
-        file_path='/does/not/need/to/exist/for/testing',
+        environment=SimpleNamespace(total_memory=1),
+        _process_initializer=touch_file,
+        file_path=str(init_flag),
     )
-    caplog.set_level(logging.CRITICAL, logger="nipype.workflow")
+    caplog.set_level(logging.INFO, logger="nipype.workflow")
     workflow.run(
-        plugin=MultiProcPlugin(),
-        plugin_args={"n_procs": 2, "app_config": app_config},
+        plugin=MultiProcPlugin(plugin_args={"n_procs": 2, "app_config": app_config})
     )
 
-    captured = capsys.readouterr()
-    assert "Custom init" in captured.out
+    assert init_flag.exists() and init_flag.read_text() == "flag"