A man named Heinz Heger was being kept in the Flossenberg concentration camp during the Holocaust. He recounted observing this strange form of torture in his book The Men with the Pink Triangle. He recalls witnesses a man being tickled with goose feathers. The man was naked and was tickled in many places of his body, including the feet, armpits and legs. The man initially stayed silent, but eventually began laughing, which eventually turned into crying.
Schizophrenics are capable of tickling themselves because of their difficulties recognizing their own actions in advance. Tickling yourself is impossible in most people for the same reason that punching others seems half as forceful as it really is — your body offsets your own force. When you tickle yourself, your brain offsets that force and doesn’t respond the tickling.