without testing the theory .. it's probably the $time input parameter available to designer graphs. assuming you have designer it wouldn't be much work to recreate the example in the video (use $time as the blend alpha input)
I'm trying to make a simple brush where the colour blends smoothly from black to white along the length of the stroke. Likewise, I'd like to apply brush size in a similar way (large to small in my case). According to the documentation, there should be a "time" attribute for some dynamic brush assets to achieve this. I…