In case you are asking for the definition of the word "imposition," it's what is done to place pages into "printer's spreads" (as opposed to "reader's spreads, which you have when you compose the book in InDesign). In short, if you pull the staple out of a magazine and see that the first page and last page are printed on the same sheet of paper (first on the right, last on the left) and on the opposite side of that sheet are the second (on the left) and the second-to-last) to it's right). That's what imposition means, and it's normally not something that the composer of the book is asked to do because it requires choices that affect production that only the production people will know, and the composer would have no way of knowing.
Most bindings you see are saddle-stitch or perfect-bound, and you would probably not notice if the pages met at the spine, fell just short of the spine or had some crossover, because the spines are all tucked together. Where you would want bleed at the spine are spiral or wire bound books, where the pages aren't connected at the spine, but separated by the coil, comb or what ever is holding it together. That's why most people think it doesn't matter with perfect or saddle.
The method you have read about will turn this:
…into this:
I think the problem you are having is that when master-page items are set to bleed off of the page boundaries, master-page items are "sticking" to the pages that they shared a spread with, like this:
One way to fix this is to leave the splitting of spreads until the very last step, after you are sure there are no more changes to the layout. Then, split the pages on a copy, and if you need to go back and tweak something for the second edition, go back to the un-separated original copy. You can override the master-page items (either individually, by Command-Shift clicking on the item on the document page, or all together, by selecting the document-page icons and selecting Override All Master Page Items from the fly-out menu), and then remove the master-page items from the pasteboards where they don't belong. It's a bit of a chore, but it gets you out of this situation, leaving you with this:
When you export to PDF, you can have this:
…and the printers can impose the individual pages with bleed if that's what they need.