Here's interesting twist I have recently found.
1. Select, say, C3:G10 range. Active cell is now C3.
2. Press Shift+↑ or Shift+↓: you see that only BOTTOM border is resized.
3. Restore original selection C3:G10.
4. Now make C10 active cell (thru Tab or Shift+Tab).
5. Press Shift+↑ or Shift+↓: you see that only TOP border is resized.
6. And if you select any cell between those "border" cells (C4:G9) and will press Shift+↑ or Shift+↓, you will see that if you press ↑, then top border is resized, and if you press ↓, only bottom border is resized.
The same thing for left and right borders of selection.