When reacting benzaldehyde with phenylenediamine, you get 2-phenyl-benzimidazoline (PBI) through an intermediate imine with the loss of water. No hydrogen is lost.
(https://www.thevespiary.org/rhodium/Rhodium/hive/hiveboard/picproxie_docs/000496725-pbi1.gif)
Upon reacting PBI with a hydrogen acceptor like a nitrostyrene (see Post 352950 (https://www.thevespiary.org/talk/index.php?topic=11875.msg35295000#msg35295000)
(Barium: "Get that double bond without borohydride", Novel Discourse)), the products are the saturated nitroalkane and 2-phenyl-benzimidazole:
(https://www.thevespiary.org/rhodium/Rhodium/hive/hiveboard/picproxie_docs/000496725-pbi2.gif)
The driving force in the reduction is the formation of the aromatic heterocycle (resonance energy) from the aliphatic nitrogen ring in the benzimidazoline. I think the reaction proceeds through some kind of hydride transfer, so no free hydrogen (radical or molecular) is ever formed.