By crouching unarmed near a brahmin and pressing the activate button you will push the brahmin over, causing it to go into a ragdoll state and getting back on its feet in a couple of seconds. You will not lose any Karma for doing this, nor will anyone or anything become hostile. However, repeating this action may cause the brahmin to die. Even after it dies, still no one becomes hostile and you do not lose Karma. You may also loot the body for brahmin steak.This is very useful in hardcoe mode to get free food. In New Vegas, each time you tip one, it counts towards the "Knocked Up" challenge.
All this time, I thought I'd explored everything there is to see and do in Fallout New Vegas. Now I realize that it was all only just the tip of the Iceberg.