I'll refrain from posting a URL, but on Mashable's site, in their post pages (not the home page), there's a floating widget to the left of the content that descends as you scroll down below the fold, but doesn't ascend above a certain point.

I'd love to replicate this, but their site is so clogged with code that I'm having a really hard time parsing apart the (javascript) scripting that's controlling this.

Thoughts or any help identifying the moving parts?