Weird behaviour of .htaccess index blocking - php

Question

I had some problems with excluding a folder from the index block in the .htaccess file in my site's root folder.
After messing around a bit, I found a solution:
<IfModule mod_rewrite.c>
RewriteEngine on
</IfModule>
I wrote this in an new .htaccess file in the folder that I wanted to be excluded. This solved my problem (unblocking that specific folder while keeping the subfolders blocked) which is great and all but as far as I know, this piece of code shouldn't do anything. How could this possibly work?
Additional information:
- the excluded folder contains a Nibbleblog installation
- root .htaccess file:
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^\.]+)$ $1.html [NC,L]
RewriteCond %{HTTPS} !^on$
RewriteRule (.*) https://url.url/$1 [R,L]
RewriteCond %{HTTP_HOST} ^www\.nyxcode\.xyz [NC]
RewriteRule (.*) https://url.url/$1 [R=301,L]
ErrorDocument 404 /errors/notfound.html
ErrorDocument 403 /errors/forbid.html
Options -Indexes

Answer 1

By adding RewriteEngine on to a .htaccess file in a subfolder, you've told Apache that the parent folder's .htaccess doesn't control rewriting in the subfolder. This is because Apache checks the folder that matches the URL first, and only then looks in the parent folder.
If you have this:
/
.htaccess
/subfolder1
.htaccess
/subfolder2
# no .htaccess file here
Then files in / and /subfolder2 are subject to the RewriteRules in /.htaccess. But /subfolder1 is subject only to its own .htaccess file, to the extent that it applies. Without RewriteEngine on, you haven't told Apache that the /subfolder1/.htaccess applies for rewriting purposes. With RewriteEngine on, you're saying, "Use this .htaccess for rewriting," even if there are no rules there.