Thanks for the replies, but you are completely missing the point of my question. These are the motorcycle hand signals to not take the right hand off the front brake and throttle.
which were based on the original car signaling method (pre turn signals)
The question was, why did they suggest motorcyclists adopt the both-handed signalling they show in the video when A) the signals differed from the car signal standards and B) the signals took hands off controls - i.e. taking hand off throttle to show you were slowing down would actually cause you to slow much quicker?