Real Dictionary

throw in
to add (something extra) at no additional cost
countable noun: When there is a throw-in in a football or rugby match, the ball is thrown back onto the field after it has been kicked off it.
phrasal verb: If a person who is selling something throws in something extra, they give you the extra thing and only ask you to pay for the first thing.
phrasal verb: If you throw in a remark when having a conversation, you add it in a casual or unexpected way.

throw-in in British English
to add as a bonus or gratuity
to engage (a clutch) or cause (gears) to mesh
noun: an instance where a sports player can throw the ball to a teammate from the sidelines because the opposing team hit the ball out of bounds