How does ORDER BY FIELD() in MySQL work internally

I understand how ORDER BY clause works and how the FIELD() function works. What i want to understand is how the both of them work together to sort. How are the rows retrieved and how is the sort order derived +----+---------+ | id | name | +----+---------+ | 1 | stan | | 2 | kyle | | 3 | kenny | | 4 | cartman | +----+---------+ SELECT * FROM mytable WHERE id IN (3,2,1,4) ORDER BY FIELD(id,3,2,1,4) The query above will result in +----+---------+ | id | name | +----+---------+ | 3 | kenny | | 2 | kyle | | 1 | stan | | 4 | cartman | +----+---------+ something similar to saying ORDER BY 3, 2, 1, 4 QUESTIONS - How does this work internally? - How does MySQL get the rows, and calculate the sort order ? - How does MySQL know it has to sort by the id column ?