The waterbike trend is already popular in France (it's meant to do wonders for cellulite) but it's still a novelty in spas over here.
And guess what? She likes it. "The hydro-bikes are in fact suitable for all physical abilities, as the cushioning effect of the water means less impact on feet and knees than you get in normal spinning classes," she writes.
It's also a brilliant workout post-pregnancy and the "steady release of antibacterial ozone streaming through the water is great for the skin."
So far, so good.
The downside? The price. It's £40 for a 45 minute session and if you want to see a difference, you'll need to go a few times. "Don't expect a miraculous transformation in one session," she warns.
Looks like hydra-spinning might catch on... with the royals and leather trouser-lovers at least.
More of Kate's sister below: