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First, every change you make on the image is stored in the History palette. That's why showing rulers for an example is not showing in the History Palette. The change must be on the image itself to be stored in the History Palette.
You take Snapshots (slice time) of the current state of the image, what is so useful is that you can save a document from your Snapshot. At the bottom of the history palette, the left icon creates new document from the selected State/Snapshot. so you'll keep everything under your control.
The number of default history states in Photoshop is 20 States, it means that if you made 22 change to your image the first two will be cleared. You can always make this number higher, do it in case you have a lot of RAM History states are hungry for RAM as their father Photoshop.
So if you are creating an Action (especially long actions) take a Snapshot as its first step, so you can always go back to your original by clicking on the Snapshot in the History palette and not by going back in the History's states.
When working with the history brushes tools keep in mind that they will not paint with foreground COLOR. Instead they paint with selected State/Snapshot from the History palette.
So to set the source (to paint from) of the History brushes, go to the History palette; at the left of the Snapshot/State tick the box. By default the source of the History brush is the first snapshot. How did that happened?
Now in the History options there's an option that says:
Automatically create first Snapshot. And that is when opening the document. This is where it takes it source.
Another option is there called:
Allow Non-Linear History: this allow you to delete a state without deleting the states that comes after it, very useful, keep it ticked.
What's the Difference Between the History Brush and the Art History Brush?
The History Brush, paints with a copy of the selected State/Snapshot.
The Art History Brush paints with artistic strokes of the selected State/Snapshot.
How useful are History Brushes?
Well they are pretty powerful in jumping back to history and paint with that State/snapshot, that is to create some real work of art effects, lets say you want to experiment with a filter or an adjustment, apply the filter/adjustment and then go back to the State/Snapshot and make it the source for the History brushes and paint. Of course you have to choose your suitable brush. The advantage of this that you can paint back to your original before applying the filter so you can apply the filter on particular areas on your image.
The Art History Brush:
The Art History Brush will paint artistic strokes from a State/Snapshot
Style: is to choose the shape of the artistic stroke.
Area: determines the area of the applied stroke in pixels.
For Tolerance controls strokes spread duo to the color in the selected State/Snapshot.
Example:
Open a colorful image
Fill the Layer with Black
Choose the Art History Brush >> set the source to the Automatically created Snapshot
Customize your brush especially the Style Option
Paint over Black to get some awesome results.
And now with the new brush engine in Photoshop 7.0 and the various options to the Brushes imagine what you can do. The only limit is your imagination.
Another cool thing is that you can stroke a Path with the History Brushes generating fabulous effects. Go to the Path palette menu/select Stroke Path and from Tool choose any of History Brushes. Keep in mind that the settings defined for the History Brushes is applied to the Path.
Take advantage of the History while it lasts, because History is gone with the wind when you close your document. Or you can always create History.
Cher compatriote, they do seem to have it over here: just pick a child and it's got the nose of its father, the eyes of his mother, the character of some great-great aunt and usually, there is even more than one possible version to choose from...
Great info Ammaro [righton] thanks. I have never tried this tool before but now that I have I can see the possibilities. How about coming up with a challenge for us so we can get some experience with it
Great information Ammar [righton] I converted it to PDF, because it's still one of those aspects (history brush) that I'm not familiar with. I think that your background information can help me a lot to have a better understanding of the use for it [righton]
Thanks Ammaro for this very valuable information.[righton] Really appreciate you taking the time to detail same and now will have to make some time to play!
Erik, Pierre. Rick, Wendy & Gaussian my man. Thank you guys. i'm thinking of a challeange, if someone had an idea please be my geust, this would be whole lotta of fun.
Good history, Better Future 
Linear Mode
