Breguet No. 28: Equal Segments
The two animations below show the mainspring being unwound and wound from the perspective of the barrel.
The first (left) animation shows that after the mainspring has been wound about one turn the crown on the arbor wheel butts agains the crown on the barrel wheel and further winding is impossible.
The second (right) animation shows what happens if the crown of the arbor wheel is cut back (shown here as a simple notch). Now the mainspring can be fully wound and the weight locked. Note that at the end of this animation the boss on the barrel wheel has only just reached the annulus and the mainspring can probably be wound one more turn.
Winding with a notch
Assuming, by an act of God, it is possible to fully wind the mainspring, then the following animation shows that the mainspring will unwind successfully. During unwinding the notch is irrelevant.