You may have to use the split line command to separate the single surface of the round part of the handle into two separate surfaces.
Then you can use the a plane to cut the surface.
And now you have two separate surfaces. I think you can apply a force to just one of those surfaces in a specific direction.
In your 3rd .png, choose "selected direction" instead of "normal". Select a plane and choose accordingly, such as a top plane and normal to plane, if your part is oriented in that way.