What are the Inverse Battle Rules?

Question

I don't have 3DS and XY.
I'm playing a lot in Pokémon Showdown and there are somethings that doesn't exist in Gen V that I play to in my DS.
There is one that I saw, it's the Inverse Battle and I don't know the rules and how to play it, there are some tournaments of that battle type and I cannot enter it.
I don't know if this is from XY or from a game created by Smogon or another person..

I want to know all rules about the Inverse Battle please.
Thanks.

Answer 1

Read Me!

It's basically a type of battle where all the type weaknesses/resistances are reversed. For example, usually Water would be super effective against Fire, but in Inverse Battles it's vice versa - Fire is super effective against Water. A whole type chart is in the above link.

That's all there really is to Inverse Battles. Everything else is generally the same. Note, moves with types that would normally do 0x damage is turned into 2x instead of 4x in Inverse Battles.

Hope I helped. :)
Source: Above Link and Experience

Answer 2

Basics:

Any type that would be resistant or immune to another type is now weak to that type instead; also, any type that would be weak to another type is now resistant to that type instead.

So by that rule, this would be the type-effectiveness chart:

For other details and smaller rules, here's my source.

Answer 3

they are battle where type matchups are reversed ex:grass is super effective on fire.
source: personal experience I played one