How std::bind works with member functions

I'm working with std::bind but I still don't get how it works when we use it with member class functions.

If we have the following function:

double my_divide (double x, double y) {return x/y;}

I understand perfectly well the next lines of code:

auto fn_half = std::bind (my_divide,_1,2);               // returns x/2

std::cout << fn_half(10) << '\n';                        // 5

But now, with the following code where we have a bind to member function I have some questions.

struct Foo {
    void print_sum(int n1, int n2)
    {
        std::cout << n1+n2 << '\n';
    }
    int data = 10;
};

Foo foo;

auto f = std::bind(&Foo::print_sum, &foo, 95, _1);
f(5);

• Why the first argument is a reference? I'd like to get a theoretical explanation.

• The second argument is a reference to the object and it's for me the most complicated part to understand. I think it's because std::bind needs a context, am I right? Is always like this? Has std::bind some sort of implementation to require a reference when the first argument is a member function?