I was thinking of values smaller than -1, so that for example -3 would select the file itself and the previous two files.
Since the value of 1 only selects the file under the cursor, then -1 would still do the same with my suggestion, but it should only be for values smaller than -1 that the cursor should be moved (to the item before the last selected item) - this way the current implementation of -1 can coexist with my suggestion.
The description in Help could be:
Change selection of the next x files from the current cursor position. Use negative values smaller than -1 to change the selection of the previous x files.
Use -1 to change the selction of the file under the cursor without moving the cursor
Currently the -1 parameter is not mentioned in the Help.