Version 2.0 or newer
This functionality is obsolete. Use CTRL+drag instead as explained in Add or remove columns by dragging.
Versions prior to 2.0
If unticked, the frames are added on the right side, when you press SHIFT+right arrow, or removed from the right side, when you press SHIFT+left arrow.
If ticked, the frames are added on the left side, when you press SHIFT+left arrow, or removed from the left side, when you press SHIFT+right arrow.
Note
This functionality uses the variable AFCS_AddRemSide.