I have seen the crank and/or cams move due to valve spring pressire alone. On motorcycles I have used vice-grips often to prevent such movement. Ford seems to believe that the spring pressure on 1 cam will cause the engine to rotate and as the engine is an interference design the valves will get bent should that happens. I suppose that if they didn't have that warning DIYers and techs might just rotate the engine with the cams immobilized and bend a few valves and then try to get Ford to warrantee it.