Today, as a default, multithreading. But that's an implementation detail. Actually, Bonobo does not support coroutines (as in asyncio coroutines) so it would be a lie to market it this way. The plan though is to allow to use coroutines/futures in the future, for specific reasons (like long running/blocking operations where keeping output order tied to input order is of no importance). Still, there is a lot on the roadmap before this becomes a priority.

I note that I still have a lot of work explaining in simple terms what is actually bonobo, without falling in the trap of "overgeneral description".