When they spot a noob feeder in one of their lanes, they give them helpful advice. Be careful. Stick near your tower. They can stun and kill you in one second...just stay behind your creeps and level and you'll be fine. They may even go into the lane with them, or offer a lane swap, so that the player can watch and play with someone supportive.
Pretty soon, your team is caught up to theirs again. You aren't dealing with a Twitch who is Level 18 while the rest of you are around 12-13. You roll their team. All thanks to a little text.
The trainer's arch-nemesis is the flamer, who is more interested in showing that he can abbreviate "what the ****" than in winning.
A noob who listens and learns. He's not going to win you the game singlehandedly, but with the right leadership and teaching, he changes his game around, surprising the other team who though him to be a feeder into an actual threat. Unable to compensate their team breaks down.
And...eh, as much as I hate to admit it, I've seen it work
The RL tank:
Throughout the entire game he's throwing insults at the opposing team. Mocking them for every mistake. He strikes at them on an emotional level. So much so that in a team fight they focus on him exclusively. They HAVE to take him down. They have to shut him up. But, while they're doing that, you destroy an inhibitor. Usually plays a tank with oodls of HP or armor or both. Many times, he escapes, only to further enrage them.