Allows you to quickly navigate up levels in a web site address. Similar to the Uppity add-on for Firefox.
When you click on the Upshift icon, it will pop up a list of new addresses that you can select, which all go increasingly higher in the address of the site you are currently viewing.
You can use shift+click to open the address in a new window, or ctrl+click / middle mouse click to open the address in a new tab. There is a keyboard shortcut, ctrl+up, that allows you to go up one level in the current address.
Upshift also provides a list of recently visited addresses on the same site (this requires permission to access your browsing history).