there are nerves that service the chin and jaw and can be affected if the chin is attacked. I believe some nerves are in the front of the chin and others are near the hinge. If these are struck directly they can, i believe, cause you to blackout.
As a similar example, when I was training capoeira I accidentally kicked another student on the cheek next to the nose. This affected the maxillary nerve that services the upper teeth. She rolled on the floor and sort of blacked out. She said that she was aware of the people crowding around her but she could not open her eyes or respond to anyone for a minute or so. She also said that her upper teeth on that side of her mouth felt like they were on fire and it was extremely painful. There must have been some swelling that lasted for a while and continued to affect the nerve because her teeth continued to be sore for a few days, even tho the teeth themselves were not struck. Needless to say I felt terrible for having done this.
Also, a dislocated/broken jaw would be extremely painful and I suppose could end many confrontations.