Infants sleep longer and more deeply when placed to sleep on their tummy. This deep long sleep is thought to be due to reduced arousal responses. Reduced arousal responses effect the infants ability to arouse from a deep sleep when they need to protect themselves. An example of when an infant needs to arouse from a deep sleep is when they need to clear their airway after they have vomited or had reflux or if they forget to take a breath or there is too long in between breaths.
Arousal is an important protective response to life-threatening challenges. Tummy sleeping significantly reduces arousal and therefore reduces an infants protective mechanisms and this is why back sleeping is safer especially in the first 6 months.