Why does creating a mutable reference to a dereferenced mutable reference work?

I understand you're not allowed to create two mutable references to an object at once in Rust. I don't entirely understand why the following code works:

fn main() {
    let mut string = String::from("test");
    let mutable_reference: &mut String = &mut string;
    mutable_reference.push_str(" test");
    // as I understand it, this creates a new mutable reference (2nd?)
    test(&mut *mutable_reference);

    println!("{}", mutable_reference);
}

fn test(s: &mut String) {
    s.push_str(" test");
}