The watch deactivates most features and locks as soon as you take it off of your wrist. You'd need to re-enter your password to unlock it. so it is unlikely that throwing it would trigger the feature.
Also from their initial demo, the fall detection uses more input than just a fast falling motion. Apparently humans fall in a specific way that the watch detects
Hopefully it is better at detecting hard falls than my series2 was at detecting the beginning of a swimming workout... I'd be driving down the road at 30 mph brushing my hair and my apple watch would ask me if I was starting a swimming workout. And I had to turn the knob to unlock to tell it no because it automatically locks you down when a swimming workout starts.
Also from their initial demo, the fall detection uses more input than just a fast falling motion. Apparently humans fall in a specific way that the watch detects