I was discussing the video with someone else a few days ago.
Here's my (suspected) take on it.
The behavior is a reward, not a 'command'.
When I was first learning about rewards, the trainer described primary and secondary rewards. Primary are basically the things needed for survival - air, food, water, sex. Maybe one or two more, but basically a very small number of things that every species needs to survive. Secondary are things like play, attention, etc. I remember the class discussing that obviously it is cruel to take control of an animals air or water to use them as rewards, and you can't use sex (until now?), so food is the primary reinforcer that is easiest to train with.
I assume, this is a behavior the dog greatly enjoys, probably touches (pardon the pun) on the sexual desires, and therefore is a very high-value reward for the dog.
Not one I would ever use, but as I said, that is my take on the situation. I would love to hear Jean Donaldson's response.