How can a function run "as if" on a new thread without doing so?

Per [futures.async]/3 bullet 1 of the C++ Standard, when a function f is passed to std::async with the std::launch::async launch policy, f will run "as if in a new thread of execution".

Given that f can do anything, including loop infinitely and block forever, how can an implementation offer the behavior of f running on its own thread without actually running it on its own thread? That is, how can an implementation take advantage of the "as if" wiggle room the Standard provides?