.htaccess gurus take a look please
 

I have a site where I have had to put a bunch of IP`s and sites on the forbidden list.I was just looking at my log and that is a lot of wasted traffic daily so I want to at least show them some ads on the 403 page. I found the following .htaccess rule on another forum:
I understand the first two lines.I already have a 404 rule set up but obviously if I just enter the 403 rule they wil not get my custom 403 page, just the generic 403 error. So some guy on the forum suggested the above rule.But everything below the # sign is beyond my comprehension :1orglaugh
My guess is that if there is a 403 or a 404 it allows only the 404/403 files to be accessed.
I particularly dont get the Deny IP`s.

I believe it's saying if the request URI is your own error403.html (or 404.html) then it is setting an environement variable named "allowit"
That way, it can still FULLY deny access to a list of IP's, UNLESS it's also requesting your error pages.
So, a denied IP trying to get to your index.html will be DENIED.
A denied IP trying to get your error403.html will be ALLOWED.