What happens when ORDER BY is given a number?

I just started learning MySQL, and the book I'm reading has the following example:

SELECT last_name, first_name, birth FROM president
ORDER BY IF(birth IS NU开发者_运维知识库LL,0,1), birth DESC;

What I wish to know is how those numbers (0 and 1) affect the behavior of ORDER BY--when you give ORDER BY a number, what does it do with it?

A hint I found is that random ordering is done the following way:

SELECT * from TABLE ORDER BY rand();

But I still don't know why it works.

It orders the data so that the rows where IF(birth IS NULL, 0, 1) returns 0 appear before the rows where it returns 1.

You might care to notice that this is quite different from what happens if you write:

ORDER BY 1;

In this case, it orders by the first column in the result list. This is obsolescent syntax, but still required by the SQL standard.