The episode reveals the origin of Jack’s "breathe" calming technique, which he learned from his comrade Donnie Robinson in Vietnam. Robinson used the method to cope with the fear of his own father's abuse, and Jack later passed it down to his son, Randall.
Despite a heart condition (tachycardia) that grants him a medical exemption, Jack convinces a doctor to help him fake his way into enlisting so he can protect Nicky, who has written home in despair. [S3E4] Vietnam
In the episode ( This Is Us , Season 3, Episode 4), the narrative uses a unique reverse-chronological structure to explore Jack Pearson's origin story and his relationship with his younger brother, Nicky. Reverse Timeline Highlights The episode reveals the origin of Jack’s "breathe"