What is the name of this effect?

I want / need to do this effect for a site, where when you click on a menu item like "work" it takes you to the page for work but then when you click on the sub menu options under "work" it uses jquery to give you the content without actually changing the url. I found the effect here, where the client showed it to me: http://www.sb-architects.com/our-work/

If you click on any of the top level items you get a second level where it changes the url, but anything under work, just filters on to the page some how but I can't figure out how. The site uses it inconsistantly and I would be careful not to do that.

I looked at the site's js but I can't figure out what script is doing this effect. It's not any of these as far as I can tell (unless I am wrong):

http://iscrolljs.com/
http://demos.flesler.com/jquery/scrollTo/
http://packery.metafizzy.co
https://github.com/dreamerslab/jquery.actual
http://imagesloaded.desandro.com/

Any help?

Thanks,
Rebecca