Nudging Selection Start/End Points
Start and end points for selections can be moved by nudging them.
To move a selection start or end point by the Nudge value:
2. Make the initial selection with the Selector tool.
3. Do one of the following:

While pressing Option+Shift (Mac) or Alt+Shift (Windows), press Plus (+) or Minus (–) on the numeric keypad to move the selection’s start point by the Nudge value.

While pressing Command+Shift (Mac) or Control+Shift (Windows), press Plus (+) or Minus (–) on the numeric keypad to move the selection’s end point by the Nudge value.