What is this DBC thing and how does it work?

question

According to S04:1454...

    In the normal course of things, C<CALL-VIA-DBC> follows these steps:

        1. create an empty stack for scheduling postcalls.
        2. call all the appropriate per-class C<PRE> submethods,
            pushing any corresponding C<POST> onto the postcall stack.
        3. call all the appropriate per-method C<PRE> phasers,
            pushing any corresponding C<POST> onto the postcall stack.
        4. enforce DBC logic of C<PRE> calls
        5. call the method call itself, capturing return/unwind status.
        6. pop and call every C<POST> on the postcall stack.
        7. enforce DBC logic of C<POST> calls
        8. continue with the return or unwind.

In light of that, what should this code do?

class A {
    method foo($x) {
        PRE { $x < 10 }
        say "A.foo called!"
    }
}

class B is A {
    method foo($y) {
        PRE { $y < 5 }
        say "B.foo called!"
    }
}

B.new.foo(3);

class C is A {
    method foo($z, $w) {
        PRE { $z < $w }
        say "C.foo called!"
    }
}

C.new.foo(7, 9);

What I'm really asking is what "all the appropriate" means in point 3.
I'm not sure I see how DBC could work without a tighter enforcing of the
more basic Liskov properties than is the case in vanilla Perl 6.