We have had to use it before and we were able to get him on the ground and control him until he calmed down and he wasn't hurt!
I worked with adolescents with special needs and found aiki really useful for defusing potentially explosive situations. Obviously, the principle of 'no harm' was of primary importance and I found the best techniques were the most subtle, just deflecting energy by tenkan movements which were not recognised as a threat or attempt to control worked just fine. I had less success with outright control movements, especially when dealing with people who had Downs Syndrome. As Robyn says, aggression is not characteristic of the syndrome, however stubbornness is. When at a motorway service station I was with a young man who sat on the floor and decided he was not going to go anywhere and there were 15 other people in our group waiting to move out, there was no way, short of lifting and dragging, that I could get him to move. His acceptance of the pain involved in any technique I was prepared to use was phenomenal so in the end we all waited until he was ready - a long time!
I think waiting was the aiki technique of preference.