So what do each of these sliders do?
Size sets the size of the brush and ranges from 0.0 to 100.0.
Feather set a gradual falloff for the adjustment from the edge of the brush to the surrounding pixels and ranges from 0 to 100.
This is how the brush appears in your image. When there is a + in the center it applies the adjustment (brush A or brush B) and a - means the Erase brush is active and the adjustment will be removed. Size and Feather can be controlled by the sliders or by using keyboard shortcuts.
To increase the size press ] and to decrease press [. Add Shift to change the feather amount: Shift [ to decrease feathering and Shift ] to decrease feathering.
The next setting, Flow, controls the rate at which the adjustment is applied with each brushstroke. This gives us the ability to slowly build up and effect within a given area. Here's an example of single brushstrokes at different flow rates.
Flow can also be set from the keyboard which allows you to adjust on the fly while keeping your brush in the image. Press 1 for 10, 2 for 20, and so on. Zero gives you 100. If you press two number key rapidly in succession you will get that setting. So 1 then 5 gives you 15. But don't wait long between the keystrokes or 1 and 5 will give you 50 (1 for 10, then 5 for 50). If you want a value below 10 press 0 first. So if you want 5 press 0 then 5.
The next setting is Auto Mask. When Auto Mask is turned on it will detect changes in the underlying pixels and restrict the brushstroke to similar pixels. When it is off then brushstrokes will not be restricted.
The last setting in this section is Density. This puts an upper limit on the amount of the adjustment applied and ranges from 0 to 100.
Density interacts with Flow. For example, if my Flow is 30 and my Density is only 50 no matter how much I stroke a particular section of my image the adjustment will never be applied any more than the Density limit set for the brush.
The last section of the panel contains an On/Off switch on the left. Click on this to toggle the brush adjustment on and off. This is similar to a before and after for the brush. To the right are Reset and Close. These will let you start over with a brush or close the panel when you're done.
There are a few more useful tips when working with the Adjustment Brush. When you start a brushstroke a pin is placed in your image. It looks like a black dot surrounded by a grey circle (the active brush). When the brush is not the active brush it's pin will be just a grey circle. Whenever you hover over your brush's pin the overlay color will appear to show you when you have applied the brushstroke.
You will also note that the cursor changes to a vertical bar with left and right pointing arrows. When you drag this cursor left and right you can change than Amount slider and therefore the amount of the adjustment applied.
Another way to see your overlay is to press O. This will toggle the visibility of the overlay. I find it useful sometimes to apply brushstrokes with the overlay on so I can see where I am painting. By default the overlay is red. Normally this is fine. However, it can present a problem is you are applying an adjustment to a red dress. It's hard to see red on red.
By pressing Shift O you can cycle through four different overlay colors: red, green, white, and grey.
Still hungry for keyboard shortcuts! You recall that you can set up brush A, brush B, and an Erase brush. When working with either A or B, press / to switch between the two. Holding down the Option key (Mac) or ALT key (Windows) will present the Erase brush. Let go and you are back to the previous brush you were using.
Well there's quite a bit packed into this panel. I hope you discovered something new about the Adjustment Brush. So get out there and be brilliant!