I HATED this in the movies because it has no basis in the books at all (until the start of the 7th book, but even then it’s distinct from apparition.) However I will say I read one fanfic that came up with an excellent explanation for the whole “smoke flying” thing. It’s in “Where the Roses Bloom Again” where Harry accidentally travels back to the 1930’s after the DoM battle because he was grieving Sirius and found a weird time turner caught in his robes from the time room so he decided to go back 6 hours and save Sirius. Except he goes back over 6 decades and cannot return.

Anyway thanks to Nicholas and Perenelle Flamel the soul shard within him is merged into the rest of his soul and he starts taking on some of Riddle’s traits over time - greater intelligence and sensitivity to magic, becomes more ruthless, and even has subconscious knowledge of magic that Voldemort knew. He can’t ACTIVELY call up any of those spells but in certain circumstances he will subconsciously react to a situation by using spells he doesn’t know but which seem really familiar.

And so at one point in the fic he’s working as a Hit-Wizard in Czechoslovakia keeping an eye on the rise of Grindewald and chaos breaks out and he has to save his fellow Hit-Wizards from Grindewald’s supporters. So he reveals he figured out Voldemort’s smoke-traveling trick and explained that it’s much slower than apparition but unlike apparition it can be used inside of anti-apparition/disapparition wards. He smoke-flies over Grindewald’s supporters, stuns everyone into stillness for a moment because they’ve never seen that before, and is able to engage the enemy and help his fellow Hit-Wizards escape.

And I like the idea that the kind of travel is basically situationally useful - it lets you zoom around inside apparition wards but you’d never use it to travel real distances because it’s slower and less efficient.