I have 2 retriever mutts and a miniature schnauzer. The alpha has put the schnauzer on her back a number of times without harming her. It's just a way of establishing superiority.
I also put them on their backs when they forget who's boss. I am not rough, but firm and I don't growl or threaten. I just stare into their eyes until they submit. Then I let them up. They remain friendly (sometimes overly so) and their main treat in the morning is to roll onto their backs for tummy rubs as part of their morning petting.
My 0.02