Paul,
The secret is to have not one, but many wand strokes to suit the soil conditions, the carpet type and colour. And often the wand stroke you use is dictated to by what chemical you have applied, in what strength and how long it has been on the carpet.
I've seen CC's who no matter what carpet they are cleaning, or how dirty the carpet is clean it exactly the same because they have not developed an awareness of different soil conditions even on the same carpet.
Simon