I just wear it on my wrist like usual. I know the spinning tracking is very much an estimate as there are a lot of unknown factors the tracker doesn't have access to. I would say the estimation is based more on HR data then movement for this activity, so I'm not sure moving to your ankle would make much difference.
The only reason to wear on ankle is to increase step count, and if you do that I would suggest not putting Ionic into Spinning exercise tracking mode.
Aria, Fitbit MobileTrack on iOS. Previous: Flex, Force, Surge, Blaze
I used it for a spin session yesterday - worked fine. As with my Surge I had it on my wrist and it was fine there, I think the advice is to wear it slightly higher up the wrist than normal but I didn't bother. Seemed to register heart rate pretty accurately - all my intervals were easily identified. Pleased. I wonder if it would auto recognise a spin session?