Basically what happens is it checks the refer (what page the visitor is coming from) and if it matches either Digg.com or StumbleUpon.com then it will simply welcome the user and ask them to give you a positive vote if they like the page.

The plugin only serves as a reminder to help people to either digg or stumble your post. It is still up to the user to click that icon to give you the vote. Increase Sociability is just a smart way to increase your traffic from social media sites, and ultimately to increase your visitors/sales/revenue !