In the latest game release, Zwift introduced a new “feature” and pair of socks. These are the new fire socks. The fire socks are unlocked at level 50, and are already unlocked for those who are level 50+. Zwift actually re-used the trail of fire effect from the 2019 April Fool’s.
How much power required?
When using the socks, you will also see a trail of flames behind your avatar when doing ~ 150% of your FTP. I am not sure what the exact number is, but it is somewhere very close to 150% of your FTP.
This does work in events which can be quite annoying for riders who don’t appreciate the over gamification used in this case. I personally don’t think that turning this on even during events was the best move, but I suspect Zwift will likely be changing this in the coming few weeks.