The scorching slap awakened the unconscious young man, but he did not immediately open his eyes. Instead, he used his ears to perceive everything around him.
This is something trained in special training. Opening your eyes immediately gives interrogators the impression of "this guy is in good shape," leading them to use more violent methods during the interrogation.
However, if you open your eyes more slowly, it makes these interrogators feel that he may be nearing his limit. To obtain crucial confessions, they won't resort to severe punishment, in case they accidentally kill him, which would be a problem.
Besides, listening to everything around him allows him to get a general understanding of the current situation after opening his eyes.
In fact, physical interrogation isn't particularly unbearable; it's just some pain. Spies dispatched abroad to carry out tasks must pass the counter-interrogation course, which includes being beaten to extract confessions.
