action_tailrecurser condition explanation

[ignus2]

Hi,
We're using rxcpp to send messages between components in our app, and we are running into some issues with the recursion, and nailed down our problems to action_tailrecurser, where we couldn't figure out the logic behind it.
Specifically action_tailrecurser::operator()

    inline void operator()(const schedulable& s, const recurse& r) {
        if (!f) {
            std::terminate();
        }
        trace_activity().action_enter(s);
        auto scope = s.set_recursed(r);
        while (s.is_subscribed()) {
            r.reset();
            f(s);
            if (!r.is_allowed() || !r.is_requested()) {
                if (r.is_requested()) {
                    s.schedule();
                }
                break;
            }
            trace_activity().action_recurse(s);
        }
        trace_activity().action_return(s);
    }

We can't really understand this condition:

            if (!r.is_allowed() || !r.is_requested()) {
                if (r.is_requested()) {
                    s.schedule();
                }
                break;
            }

What is this trying to do? Can someone explain it in plain english please? :)
Thank you in advance!