Smoothing parameters

Kernel width.

As the smoothing is done in time, this is the width of the smoothing kernel in time. Can be any odd number from 5 to 15. (Other numbers are impossible because of the structure of the algorithm and the speed optimizations)
Smaller number saves the processing time. Higher number makes the smoothing more accurate (from the math point of view), but it does not smooth stronger and does not result in "motion blur". That is because this parameter means the maximal kernel width, while the actual kernel is dynamically reduced if a scene change is detected.
So, don't worry about blurred motion even if you set 15. Such an effect will appear if you set too high the smooth strength.
Set 13 or 15 for high noise video. Set 5 or 7 for faster processing or debugging with preview.

Intensity smooth strength.

The smoothing is made as the weighed average. That means, for a current pixel in time, its intensity is compared to the intensities of all the other pixels in the kernel. The smaller is the difference the more weight is assigned to the corresponding pixel. If all pixels have the same intensity difference with the current pixel then all of them are assigned the same weight.
The smoothing strength defines the effective width on intensity scale for the weights assignment.
If you process a video with high noise (e.g. VHS source), set this parameter to 9 or higher.
Do not set it too high, otherwise you may get the edges of moving objects blurred, particularly in low contrast scenes.

Correction for color smooth strength.

Color components of the pixel can be smoothed with a different strength than the intensity. I noticed that normally it is better to reduce the smooth strength for the color. You can achieve this by making this parameter negative.
Note that this number is not simply added to the "Intensity smooth strength".
If you use an even number for the "Intensity smooth strength" then the correction for color smoothing can not be less than -5 (-4 for MMX_only version).
Otherwise it can not be less than -10. This condition arises from the speed optimizations.