What's the grammar related to the "in you go" here like why's an order different? Why's "in" at the beginning?

Native speaker’s answer
Rebecca
This sentence is a more informal version of the imperative "Go in," so the structural changes create a softer tone and directs the person in which direction to go in first. So this is a phrase you'd often use to direct children or people who may be fragile or sensitive, or if your job requires much care, then "in you go" or "out you go" would be gentler to use than "go out," or "go in!" This isn't used with other directions, mostly with "in" and "out." Ex: We're going to the shops now, Henry. In the car you go. => saying this to a child Ex: Go left here at the traffic light. Ex: Out you go. It's time for your walk, puppy.