Docs: add description of Indirect Condition problem

docs/apworld_dev_faq.md

@@ -43,3 +43,26 @@ A faster alternative to the `for` loop would be to use a [list comprehension](ht
 ```py
 item_pool += [self.create_filler() for _ in range(total_locations - len(item_pool))]
 ```
+
+---
+
+### I learned about indirect conditions in the world API document, but I want to know more. What are they and why are they necessary?
+
+The world API document mentions how to use `multiworld.register_indirect_condition` to register indirect conditions and **when** you should use them, but not *how* they work and *why* they are necessary. This is because the explanation is quite complicated.
+
+Region sweep (the algorithm that determines which regions are reachable) is a Breadth-First Search of the region graph from the origin region, checking entrances one by one and adding newly reached nodes (regions) and their entrances to the queue until there is nothing more to check.
+
+For performance reasons, AP only checks every entrance once. However, if any entrance's access_rule depends on region access, then it is possible for this to happen:
+1. An entrance that depends on a region is checked and determined to be nontraversable because the region hasn't been reached yet during the graph search.
+2. After that, the region is reached by the graph search.
+
+The entrance *would* now be determined to be traversable if it were rechecked, but it is not.
+To account for this case, AP would have to recheck all entrances every time a new region is reached until no new regions are reached.
+
+However, there is a way to **manually** define that a *specific* entrance needs to be rechecked during region sweep if a *specific* region is reached during it. This is what an indirect condition is.
+This keeps almost all of the performance upsides. Even a game making heavy use of indirect conditions (See: The Witness) is still significantly faster than if it just blanket "rechecked all entrances until nothing new is found".
+The reason entrance access rules using `location.can_reach` and `entrance.can_reach` are also affected is simple: They call `region.can_reach` on their respective parent/source region.
+
+We recognize it can feel like a trap since it will not alert you when you are missing an indirect condition, and that some games have very complex access rules.
+As of [PR #3682 (Core: Region handling customization)](https://github.com/ArchipelagoMW/Archipelago/pull/3682) being merged, it is also possible for a world to opt out of indirect conditions entirely, although it does come at a flat performance cost.
+It should only be used by games that *really* need it. For most games, it should be reasonable to know all entrance → region dependencies, and in this case, indirect conditions are still preferred because they are faster.