It's not an answer to you directly but more of an observation: The effect for this page is very laggy on a 1-year old MBP, and makes the text barely readable until a certain scrolling point is reached. I do not understand the purpose of using the effect here or in 9/10 cases I see it on sites. It introduces pointless visual candy with usually zero or negative practical benefit. Makes me think of the DHTML days.

unfortunately we wrote it on an Macbook Air, where it worked pretty good. It lags horribly on an MBP retina, we might fix that

You can have similar effects with CSS, but in this case it is JavaScript: http://alcatraz.io/javascripts/parallax.js