Motionless is the secret, if you can lock on to a focal point you will obtain motionless while Standing your on the right track,
If you cant stand motionless I reccommend that you keep trying untill you even Start SNT,
hows your movement when in the stance, Do you feel motionless? if so how do you maintain it?
I think your talking about "Standing Practice"
It's used so that you can concentrate on maintaining the "Tei Gong" internal contraction and work on straightening and expanding the vertebrae in your spine and finding your centre of balance.
Any problem regions where you carry tension can be isolated and targeted in standing practice , specifically thighs and shoulders , but other areas as well.
The end goal being that our back is straight and we are as relaxed as we can be in our Wing Chun stance.
When you say motionless while moving in your stance I think you may mean excessive upper body movement as you are stepping or kicking.
Things such as leaning before you move in , or shoulder movement when kicking.
Basically , it takes a lot of practice in front of a mirror by initiating your stepping from the waist while keeping your back straight.
Similarly with kicking , practice "leg raising" raise each leg alternatively while in your stance in front of the mirror.
Try to minimize any movement of the upper body as you raise the leg , specifically the shoulders.
Eventually you get to the stage where there is hardly any movement at all.
