Anything below high-hang, set the start position a moment before starting the lift. For high-hang, just down & up (i.e. dip and go) in the Burgener terminology.
The scoop does begin right as the bar passes the knees for many lifters - but ideally it shouldn't start until the bar is a bit higher - around mid-thigh. Try to find some video of Stefan Botev for a good example of this.