Step 1: The question asks which maneuver is NOT part of neonatal resuscitation when the heart rate is under 60.
Step 2: Approved interventions are ventilation with oxygen, chest compressions, and gentle tactile stimulation, all of which are safe and evidence-based.
Step 3: Tactile stimulation means soft measures like rubbing the back or flicking the feet, not forceful blows.
Step 4: Slapping the back is outdated and potentially injurious to a newborn, so it is the exception. Therefore slapping the back is the answer.
\[\boxed{\text{Slapping the back}}\]