"Play physiologically is only possible when both the ventral-vagal (social engagement) system and the sympathetic (doing) system are simultaneously activated. This allows play to be both adventurous and active, and also very social. If someone is say accidentally hit with an elbow during play, they will not get (very) upset if the ventral vagal tone is strong, but they will get very upset, involuntarily, if the ventral vagal tone is weak, even if intellectually, they 'know better.'"
