it is enabled by default on your wordpress.com blog. while it is a cool add-on, sometimes it just gets in the way of the viewers trying to scan through your page. plus it eats up precious bandwidth.
you can, however, disable it. after you log-in, go to the following: My Dashboard –> Presentation –> Extras
or go directly to http://yamz.wordpress.com/wp-admin/themes.php?page=theme-extras *
then uncheck Enable Snap Preview Anywhere on this blog, hit Update Extras and say bye-bye to Snap Preview.
*note: change yamz.wordpress.com to your wordpress blog name.