how does session data manipulation work with cookies?

[eluser]MikeW1911[/eluser]
Around 31:15 of this video: codeigniter security tutorial, the guy changes the user_id session value to the admin user id and gains full access. I'm not sure what he did to change the session value.

If I change my cookie values using a developer tool, would Codeigniter simply read the cookie values and change the corresponding session value?