I took my HP test about 10 months ago, and as far as I can remember you click as soon as the object in question is going to cause you to change speed or direction. So, in that clip you would click as soon as the lorry was in view.
For a ped crossing the road, you'd click as soon as they appear to start to move to cross the road, like the turn of their head or a change of direction.
But yeah, the most crucial thing is knowing the object will make you change speed or direction.