The history palette is just like a fantastic expansion of the edit menu: it lets you handle different states of your documents so you can always go back, it lets you save some RAM, and is especially usefull when you submit different versions of a project.
When I made my jeans boots, my file soon ended up with more than 50 layers which included masks, adjustment layers, etc...hard to handle, both for me and Pshop, and very risky.Also,I needed to be able to go back at some early states of my document, therefore I created history snapshots before every critical operation : if you look at your history palette, you'll notice it displays every operation you made
Just try it: open a document, and change its hue (Ctrl+U) to change its color, repeat twice, with different colors..(ie: red, green, yellow).Then your history palette should look like this
Now, by clicking on the different history states, you can compare, get rid of one state by draging it into the trash can at the bottom of the palette, or create a new document from it...and this is the most usefull tool, as it lets you work on a new documpent, you can just save and close your original file to be sure you dont destroy/Alter it.Also, when you work on a file that involves all your RAM ressources because it's huge, and have finished preparing it, create a new document from its current state , then flatten the layers, and work easily on it to correct colors, or do some various final retouching operations.
For instance, I want to make a pic in which I have different versions of the boots.As the boots are composed of 40+ layers, I have to flatten them all, so I take a snapshot of its current state so I can go back to it
Now just merge all layers into a new layer on top of the above as done in the previous lessons, (Ctrl+Shift+N, then Alt+Ctrl+Shift+E), and I create a new document from this state so I can work easily on it, without all the ressource taking non visible layers
Now I can save and close the original file, and work only on the recently created one, and I can delete all the layers I dont need anymore , and keep only the one I have merged all the visible layers into.The easiest way to do it it to toggle off all these layers , by Alt+clicking on the eye on the layer you want to keep, and then choose "delete hidden layers" from the layer palette menu
Now you you have a safe original file, which contains all your masks, etc, and a new file it is safe to work on.Since I had to use 4 different files to make the boots, I could not even imagine what I should have done if I had had to work with limited undos. Hopefully this will prevent you from making pics that don't look the way you want them to just because you can't go back.