I don’t know if i’m fully there with this advice. Some of it sounds like the writer wants you to be more monotone than you feel. To control the way you express your reactions.
Expressing your own emotional reactions to/in front of a child is surely important in their development.
That said, the writer’s key point about accidentally switching a child’s motivations for learning things through the parent’s actions is well put.
Yeah, I was getting a lot of management course ‘you need to offer specific positive reinforcement’ vibes…
‘Good job! And I particularly like how you released your hips to get extra power!’
Or ‘When you…I feel…because…’