on normal 35mm slr with a mounted flash
in normal view (landscape)
Horizontal is for bouncing light off wall (if there is) to create side lighting
Vertical is to bounce off ceiling (if there is)
Use both together will either bounce off higher wall of each side.
in rotated view (portrait) (once rotated, axis change)
Horizontal =same as above but on 1 side, depending where the flash is on now
Vertical is to bounce off the ceiling or floor