From Wowpedia (
http://wow.gamepedia.com/Mak%27gora#Rules):
Rules
As described in The Shattering and Bloodsworn, the rules for Mak'gora are:
-Each participant is allowed one weapon.
[Garrosh: Gorehowl, Thrall: Doomhammer]
-A blessing of this weapon by a shaman of their choosing is permitted, but not required.
[Garrosh: None, Thrall: None]
-Body armor is forbidden.
[Garrosh: No chest-piece, Thrall: Doomplate]
-Each participant must have at least one witness.
[Garrosh: None, Thrall: Durotan, Draka, Yrel, Player-Character]
-If both participants refuse to fight one another, they are banished from the clan.
-Traditionally, it is to the death, but under Warchief Thrall's rule it became a non-lethal combat, similiar to Warsong's Mak'Rogahn. Participants can choose to forgo this change. However, even when operating under the old rules, the victor can choose to spare the loser's life.
So... Yeah. Thrall cheated in two ways. First, Thrall used body armor (his Doomplate) whereas Garrosh did not even wear a shirt. Second, Garrosh had zero witnesses from his new Warsong Clan. HOWEVER, there is nothing in the rules about using special powers (like Thrall's Shamanism), but from the rule-set described above in two different novels that are canon... Thrall DID cheat!