Pete - I've just watched your little video as research for a forthcoming few days at Coniston. Don't want to preach - but:
1. You don't have the kill cord connected
2. You were letting go of the tiller - whilst this nicely proved that the engine was balanced etc - you didn't have control and hitting an unseen submerged object (e.g. branch) could have cause a sudden violent turn.
Obviously your throttle tension is set so it doesn't spring back to tick over either.
If you don't understand the implications of this please search the forum.