Click on the broken line. This way you cut the part of the rope beyond the broken line. At the end of the rope the wave will turn back. It behaves as if the wave propagating farther along the feint line would be mirrored on the dividing line. At the reflecting point the reflected wave has an identical phase to the one of the incoming wave, so it corresponds to the reflection on a free end.

Fix the right end of the rope with the “lock” icon.
At the reflection point the reflected wave will have an opposite phase.
It appears as if the wave reflected on the free end would be mirrored on the line of the static rope.