Canonically we have the friction explanation. Some think sanji just spins so fast we can't see it post ts. I don't buy it but some do believe so.
In a sbs oda mentions that it's based off his passion. Similar to haki. It's literally stronger when sanjis passion is up. Look at hell memories
Also could be a df secret unlocked by Judge.
In another sbs oda mentions devil fruit and spinning and sanjis eyebrows and that being the source of his power
All in all I think there's many possibilities but these are what we know canonically. My personal opinion is that it's his passion but also could be a df modification by judge
We've went 1000+ chapters. I want some answers Loda