throw in
throw in {v.}
1. To give or put in as an addition; to give to or with something else.
John threw in a couple of tires when he sold Bill his bicycle.
Mary and Tess were talking about the prom, and Joan threw in that she was going with Fred.
Compare: FOR GOOD MEASURE.
2. To push into operating position.
Mr. Jones threw in the clutch and shifted the gears.
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Source: A Dictionary of American Idioms