Since I have no way to mount my phone to my handlebars, I had a go withit in the radio pocket of my bib shorts. So, upper back, leaning to steer. It took a bit of getting used to. First attempt I got 5/10 stars, overcompensating to adjust my line. Second time I tried to be a bit more analogue in my steering. Came out with 8/10 stars and a ~30 second improvement. Lost of full star on the first section (“a bit rough”, cheeky). That was really annoying because I got a full star on my first run when I was all over the place!
If the phone is on your upper back and you get out of the saddle, the avatar sways left and right. Looked quite realistic I thought. =)
Overall, I think having the phone in the middle of my back won’t be a long term solution. The steering felt quite unnatural, and certainly not the “it just works” that GPLama felt in his video.
Also, it’s impossible to see the map or give ride ones/use power ups via the Companion app if it’s behind me. 
I’ll have to invest in a bike mount.