The sole purpose of hill decent is to keep the car below about 5KPH without input from the driver in an attempt to keep the car in control using the ABS system. A driver attempting to keep the car slow by the brakes will usually overuse them causing loss of traction - likewise using the clutch may cause the vehicle to run away.
I suspect that on the 1-2m gradients the car is simply not getting up to speed for the HDC to operate. For these type of gradients I suggest having HDC engaged well before the slope, the car in first gear and feet away from all pedals - the car will idle up to the crest, go over the top and if it gets up to the required 4-6kph the HDC will apply and release, apply, release the brakes to maintain the speed.
Remember the HDC doesn't know directly how steep the hill is - it is simply attempting to keep the car below a set speed - the steeper the hill the faster the car will want to go and the more the HDC will be applying the brakes until a point is reached where gravity overcomes the coefficient of friction between the tyres and ground and the car just slids - heavy footed use of brakes will bring this on earlier rather than later and the HDC is designed to delay this as late as possible - the trick is to not touch the clutch, brake or accelerator pedals - feet off!!
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