It is easy to install - just paste the code generated on at the Snap site directly above the </head> tag on the page(s) you want the feature.
The snap site gives this description:
Snap: "Snap Preview Anywhere enables anyone visiting your site to get a glimpse of what other sites you're linking to, without having to leave your site. By rolling over any link, the user gets a visual preview of the site without having to go there, thus eliminating wasted 'trips' to linked sites. "
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