Earlier is cheaper
There is a well known map of where a person can step in on the course of an emotion, and it runs from choosing or changing the situation at one end, through where you put your attention and how you read what happened, down to managing the response at the very end. The consistent finding is that the earlier you intervene, the less it costs and the better it works.
Holding the expression in at the far end is the most expensive option there is, for you and for the person in front of you. That is the clinical reason this course spends most of its time upstream, and it lines up with what lesson three says about restraint not meaning suppression.
What the research says
Gross JJ (1998). The emerging field of emotion regulation: an integrative review. Review of General Psychology, 2(3), 271 to 299
Sets out five points along the life of an emotion where a person can intervene, from choosing or changing the situation at the start to managing the outward response at the end. The pattern across them is that earlier intervention is cheaper and more effective, and that holding the response in at the last moment is the costliest option. The identifier for this paper was not confirmed for this course, so it is cited by title, journal and year only, and no figure from it is quoted.