Autostack is probably one of the least used features but one that can save you an immense amount of time. If you happen to shoot digital and have the image capture date embedded in all your images for you autostack is an extremely precise tool that you can use at the beginning of your workflow.
It doesn't really matter if you shoot sports, portraits, fashion or landscapes. Unless you are very very different from the rest of us human beings your shots of the same subject with the same lighting, framing, and perspective will usually fall within a short time period with a gap before you change your framing, lighting, subject, etc. Autostack stacks and unstacks images based on the time between images and… You can see it visually. This is fantastic and far easier than doing it manually. The big win here is when you use stack mode to review your images and get to the best ones very quickly.
Understanding why autostack is so great requires you to understand why albums are great to bubble various edits/versions/post production treatments up to the top as well. The other thing that might help are a couple of tips from me because I am an autostack/stack mode junkie. The trick is to go a little overboard with the autostack. In most cases that I have experienced you end up with everything falling neatly into really close times if they are different shots/framing/etc. with one or two that are very different that get picked up and jammed into a stack that you really don't want in a stack. Like when you do a really really quick change in framing or composition and go back to the same one you were using before - etc. If you are too OCD with using autostack not wanting anything to be stacked "wrong" you end up with not nearly enough photos that belong in the same stack actually autostacked. It is far better to go a little more aggressive an end up with an image here or there in the wrong stack. or more likely a couple stacks that are really big with two different subjects/compositions in them.
The reason for this is that it is much quicker to get things out of a stack than it is to put them together using just the keyboard. Most of the time what will happen when you use autostack to it's full advantage is you end up with two subjects in one stack - this is even easier to deal with while using just the keyboard. The magic keyboard short cuts correspond to "Extract Item" and "Split stack" under the stack menu. Look them up in the menu.
Learning to use autostack, a couple of keyboard shortcuts, and stack mode took my process to get to my selects down from hours to minutes for my commercial work. I found once it was so painless I actually had time and desire to do it a couple times and guess what - I made better decisions.