If a cable is broken under damp soil there will be a fairly small area where the voltage is 'leaking' into the ground, before the resistance of the soil 'stops' it. If we stand on it with our legs close together, the voltage potential is going to be 400v (or whatever the cable is carrying) at both of out feet, so no real issue. If a large animal with four legs stands on the patch which has the voltage with his front legs, his back legs are going to be on a patch with significantly lower potential, let's say 20v - as voltage always wants to get to zero, the remainder of the voltage (380v in this case) will flow through the animals body from the front legs to the back - straight through the heart of the animal.
That's how I understand it, anyway.