https://www.youtube.com/watch?v=a5YSJsNxPB8
Any time Rek'sai knocks up a Nunu while he's in his W causes him to be rocketed to Narnia, stuck in the same position on the map. It's essentially a knockup plus an 8 second snare on the Nunu.
Thanks for posting this. I'll take a look at our bug tracking and see why this isn't tracked or if it is, why we haven't fixed it! Hopefully I can give it a go this week!