Most critically, a particularly distressing side effect of punishment is that it can actually suppress warning behaviors. So, punishment may SUPPRESS a behavior, but it does not address the underlying EMOTIONAL state of an animal. This is critical with regards to pigs - pigs are PREY animals and are inherently wary, skittish and fearful. If we use punishment (pushing/moving/etc) when a pig reacts out of fear, we may well suppress those behaviors initially. However, the pig is still fearful - perhaps more fearful now that he has received punishment - but he has only learned to suppress the behaviors associated with that fear in order to avoid further punishment. At some point, the pig is likely to become unable to suppress the fear anymore and will suddenly react violently and without any warning signs. A pig that used to bark or rumble before snapping or lunging may now attack or bite without any warning!
I can't tell you all how many times I've heard the phrase "Being top pig really worked well for a while, and then suddenly out of the blue, my pig bit me (or my child, or my friend)!"
Hearing this always makes me sad, for both the people and the pig. Misinformation and miscommunication can lead to tragic results. Carefully consider whether your chosen behavior modification program is based on scientifically-backed principals of learning, and always consider the possible negative consequences.
AVSAB Position Statement - Punishment