The theory I hear from people who do it is that the animals die instantly in thermal shock. Would be great if someone in the know could comment; I'm certainly not endorsing this view!
The latter is not a task I expect many home cooks to be good at.
They key is to have the water at a rolling boil and have enough volume that it doesn't cool when you add a 2 lb lobster head first.
I suspect a lot of the horror stories are from people who added lobster to a pot with too little water.