Yoshinkan is derived directly from O-Sensei himself as Kancho Shioda(the founder of Yoshinkan) was his student. The main difference
between Yoshinkan and Aikikai is in the aesthetics mainly. You don't usually wear hakama until your dan rated (in the case of my Dojo, until
you have reached 5th dan). Atemi is usually a full on strike rather than a distraction. Some of the terminology is changed. Ikkajo rather than Ikkyo for instance. Most aikikai students find it very easy to slide into Yoshinkan with a little help. We've had/have several aikikai students at our dojo. In Britain, one of the best Yoshinkan sensei is Anthony Yates. You can chech him out at You Tube or on his web site. With regards to the riot police, all are trained in the instructor course at the Hombu dojo before returning to their respective districts or prefectures to train their fellow officers.