Upon examining the source code of startupgiraffe, it becomes apparent that there are multiple sections dedicated to the "background":
<section id="background-wrapper" class="full-wrapper">
<section id="first-background">
<div id="scroll-trigger"></div>
</section>
<section id="second-background">
</section>
<section id="third-background">
</section>
----
</section>
The developers have implemented a significant amount of javascript to seamlessly transition between these background sections, creating a visually appealing cohesive background.
If you are interested in replicating this effect or delving deeper into the technical details, I recommend exploring the source code of the HTML, CSS, and javascript files. Additionally, utilizing browser analysis tools such as Firebug or Chrome's console can provide further insight into the website's functionality.