"Whenever a piece leaves its initial square, one of the extra pieces can
immediately be introduced to the vacant square. So this is a double move,
comparable to castling. Should a player refrain from inserting his extra
pieces at these occasions, then he has forfeited his chance of introducing
them." - Ok. - "When castling, one of the extra pieces can be placed on
either of the squares left vacant" - Isn't that a contradiction?
Before you castle you have to move two pieces at least. That means to me,
you have already defeated your right to insert them.
What do I misunderstand