As AP Yi you have to wait till someone from the enemy team has low enough HP amount to be killed one one Q. Then magic happens. Imagine two situations:
1. 4v5 on mid lane, your team is poking enemy team, enemy team is poking your team. Why 4v5? Because you are waiting somewhere at their back, waiting for a good moment, waiting to kill them all in a chain of alpha strikes all over their team..
2. 5v5 on mid lane or anywhere else. Your tank goes in. AD carry deals dmg. Top and Jungler are trying to focus AP and AD of their team. You are going in with Q - their AD carry dies, you Q again. And again. And again..in madness..
and it works only if you know how and when to strike. AP Yi mid is op if he has blue buff. :P
I've got friend that has about 2500 wins at normals, playing since the LoL started, he's a fan of AD Yi in jungle. He buy early Sword of the Divine, goes on bot - Q into their adc - in the same moment activate SotD and Wuju to increase DMG - that's some unbeliveable damage right in enemy face. Early fed, late game monster. This tactic works till enemy finally builds some armor, then just sell SotD and replace it with something more worth in late.