This build will allow Master Yi to dodge a third of the hits, nearly every single strike, life steal half of the wonderful damaging he's doing, hit super fast, crit super hard, and with the teleport spell be able to move quickly to drop turrets and push very effectively. The ignite spell is to counter any healing such as Fiddlesticks, another Yi, or Tryndamere or anyone who decides to life steal/heal. Ignite, therefore, is purely for finish kills or for stopping someone from popping their health regen. Take care with this build though, it requires a lot of gold and will require a lot of patience early and mid game. This should shine very well late game. The Warmogs is to give him the health he needs to really put the hurt on the opposing team and not have to relly so much on hit and run. With the attack spead, damage, and critical strike chance and damage, he shouldn't have a problem with health coupled with the Warmogs.