0 Pressing [Backspace] will go back a page in the session history and [Shift]+[Backspace] will go forward. (Default in Windows)
1 Pressing [Backspace] will scroll up a page in the current document and [Shift]+[Backspace] will scroll down. (Default in Linux builds before 2006-12-07)
(Other) Any other integer value will simply unmap the backspace key. In Linux builds after 2006-12-07, the default is 2.