diff --git a/curvlinops/experimental/activation_hessian.py b/curvlinops/experimental/activation_hessian.py
index aaf9de32..039786c0 100644
--- a/curvlinops/experimental/activation_hessian.py
+++ b/curvlinops/experimental/activation_hessian.py
@@ -150,6 +150,14 @@ def _matvec_batch(
         return hessian_vector_product(loss, [activation], x_list)
 
     def _preprocess(self, x: ndarray) -> List[Tensor]:
+        """Reshape the incoming vector into the activation shape and convert to PyTorch.
+
+        Args:
+            x: Vector in NumPy format onto which the linear operator is applied.
+
+        Returns:
+            Vector in PyTorch format. Has same shape as the activation.
+        """
         return [from_numpy(x).to(self._device).reshape(self._activation_shape)]