Are you ready to enter the Pokemon Arena? Ok, simply put this is "the counter game" that most sites have in one form or another. The basic idea is one person posts a Pokemon, then the next post counters that Pokemon.
For example I might post..
Latios @ Choice Specs
252 Speed / 252 Special Attack / 4 Defense
Hidden Power Fire
..then the next poster could post..
Tyranitar @ Choice Band
252 HP / 252 Special Defense / 4 Attack
.. which could be followed by a bulky Swampert, or something. A few rules before we begin!
1) Double Team, as well as any and all evasion is banned. So are phazing moves and Baton Pass. If anyone posts these by accident just pretend the Pokemon has 3 moves.
2) OHKO moves are banned. Though they have counters in Sturdy Pokemon it's still annoying.
3) Self KO clause is in effect. If you KO your opponent through Explosion, Brave Bird or whatever and then faint yourself, you lose.
4) No Destiny Bond in general since it can be nearly impossible to beat in some cases.
5) Focus Sash is banned to prevent a game of "I switch in something faster and KO you"
6) You must be able to switch in and win 70% of the time or better.. So don't just say "I switch in and use Focus Blast three times in a row".
7) Assume your opponent can predict perfectly. You will always switch in on the worst move possible.
8) Stealth Rocks, Sandstorm, Spikes and so on are NOT in play unless the thing you are countering sets them up (and you switch in as they Spike / SR so you will not receive damage from them).
9) If two people counter the same thing (due to forum lag or whatever) the one who posted first counts.
10) All standard OU bans apply.
11) Shedinja is also banned. You're a jerk for even considering it
12) No troll sets. Something weird like Specs Tyranitar is okay as long as it looks decent, but these sets like "Tackle / Toxic Bidoof with Stick" are just pointless.