You can find the feature on the brush settings panel which can be found via the window menu
They are not cast in stone, they can be modified in countless ways and one of the more powerful features is the dual brushes, perhaps not the greatest name but it is still a super useful and can modify any of your presets in all kinds of unique ways. You can use them to add and subtract from the existing presets. Add them via the entry in the settings panel by adding a new dab from long list of dabs. As you can also combine it with scattering, spacing etc, literally millions of different combinations of preset can be created even from a fairly basic set of dabs.
Go to that panel and you will see a selection of dabs. select one, though it should be noted that not all the dabs are available for use, some will be grayed out. Select one of them and straight away you will likely see a change to the preset in the bottom preview pane.
The end result really depends on the dabs available. I love to add the diffuse / noise dabs to my settings as you can create amazing noise / grainy strokes by using high scattering, if you set it to a low setting, you basically see the standard stroke. Of course, all kinds of different designs can be used in this secondary role such as lines, rings, star designs etc but the noisy ones are my personal favorites for adding that special touch to any preset in the panel. Changing the mode also helps, color dodge etc works amazing, linear burn etc all work well with them. Many different types of paint effect can be created by changing the combo of these with reduced spacing and increased scattering.
Change the size of this new dab, perhaps go for a very small size or big size and you will see the preview change.
This is also dependent on the mode set at the top of the panel such as multiply, darken, color dodge, overlay etc (not all the blend modes are available). The linear height mode is probably the most interesting.
The count just makes it far more dense, so the count works best with low to medium values.
you can increase the size of the spacing so you can see bigger gaps between the particles or you can set it low and have a more concentrated preset.
With 100% spacing the result is a little more sparse. You can use this to spread out the design. You can push it to a 1000% which basically means you end up with very few dabs appearing as you apply the stroke
You can scatter the particles beyond the brush by using scattering and especially useful is the both axes option. If you reduce the scattering down a very low setting, you basically get a very thin brush stroke dependent on the size etc. I generally ramp up the scattering to a decent level to see the design. The scattering is great for spreading the design across the entire image so in that case set the scattering to the maximum and set it to both axes as well. To get the yellow / green effect below, set the color dynamics on and set the foreground color to yellow or green and set the hue dynamics etc to as high as possible and apply across the document. The source design for this was a very basic dot / rough stroke.
You should not avoid the mode at the top of the panel. Try out all the modes with your preset as linear height is very different from darken. It does seem odd that Adobe didn't add erase / subtract as well as difference etc to the list. There is an erase like feature if you use the brush strokes on layers and then you will see linear height works and a negative space on the brush stroke. The image below is a dodge blending mode
The same source brush stroke and settings but this time with linear burn
With the linear height the result can be very different
It should be noted that the feature depend on the original brush size, if you change that you will see more or less of the particles and if you change the spacing the combo of two brushes, the end result can vary greatly such as having pulse of brush stroke instead of a continuous stream of brush stroke.
They are even more effective if you use them on a layer and then go to the layer styles panel and add layer styles such as bevels or shadows. Also, when they're applied on a layer you can also change the blending mode of the layer and create interesting color combinations of the new presets.
It should be noted that the feature is effective with the standard paint tool but not the other tools. Sadly, some features such as this only works in one or two of the tools