reCAPTCHA Security Preference differences? (Easiest to Most secure)

[Alan Gerling]

Official description of this setting:

A slider that allows you to adjust the security preference for the site. Value of the slider ranges from "Easiest for users" to "Most secure".

-Captian Obvious

Anyone have a bit more information on what this slider setting does? 

I'm guessing it just increases the frequency of the image game challenge popup but only basing that on nothing.