Scissors
Opens a closed outline at the point you pick, so the beam starts and stops there. The shape does not change. What changes is where the little blemish at the join ends up, which is the difference between a visible seam on the front face and one nobody finds.
1
A closed outline starts and stops wherever it was drawn, which is rarely where you would choose.
2
Click the outline at the point you want the cut to begin. The cross shows exactly where it will land.
3
Same shape, same size, now open at that point. Click an open path instead and it comes apart into two.