Shift In (シフトする Shifuto suru) is a term that refer to the act of placing a Shift card on the top of a card that meet the "Shift Vessel" (剣臨器 Kenrinki) criteria in the Shift card text.
1015 Shift In
1015.1 "Shift In" means a Shift card is put onto the top of card designated in the text (Shift Vessel) from Dynamis.
1015.2 If a Shift card is played, resolved, and moved from Temporary onto the top of a card, that card is considered to be Shift In from the Dynamis.
1015.3 If several Shift cards Shift In simultaneously, unless otherwise specified, each of them must Shift In to different cards. If the number of possible card to Shift In is less than the number of Shift that must Shift In, the controller of effect that make those cards appear choose as many Shift cards as possible from among those cards, and make them Shift In (102.4).