I think it's probably confusing because its not that the "address" is updated, its that the "to" and "from" are written in code. 

Imagine there's a table where people can drop letters. You write who it's to and who it's from on the envelope:

To: Alex
From: Em

If it's yours you pick it up.

You and your friend don't want others to know you put a note in the pile so you come up with a secret code you both recognize for each other. 

The code takes the number of letters you've sent combined with your name to make some gibberish so only you and your friend know who it's from and who it's for:

To: code(10, Alex)
From: code(10, Em)