Set in Motion is a keyword action which is only used in the Archenemy format. Together with abandon it is used to interact with Schemes.
From the glossary of the Comprehensive Rules (October 4, 2019—Throne of Eldraine)
- Set in Motion
- To move a scheme card off the top of your scheme deck and turn it face up. See rule 701.24, “Set in Motion.”
From the Comprehensive Rules (October 4, 2019—Throne of Eldraine)
- 701.24. Set in Motion
- 701.24a Only a scheme card may be set in motion, and only during an Archenemy game. Only the archenemy may set a scheme card in motion. See rule 312, “Schemes,” and rule 904, “Archenemy.”
- 701.24b To set a scheme in motion, move it off the top of your scheme deck if it’s on top of your scheme deck and turn it face up if it isn’t face up. That scheme is considered to have been set in motion even if neither of these actions was performed on it.
- 701.24c Schemes may only be set in motion one at a time. If a player is instructed to set multiple schemes in motion, that player sets a scheme in motion that many times.