The Combo Move is an ability used in the game and episode "Guardians of Sunshine," to defeat the enemies that come from that game. Though Finn could not do this move while playing the game, he successfully uses it to defeat Sleepy Sam, Honey Bunny, and Bouncy Bee in real life. Afterwards, BMO gets annoyed at Finn and Jake ignoring his warnings, and starts to use the move on them.
To use the Combo Move, you must do these movements: up, down, left, left, right, right, down, spin, down, up, left, right, left, down, spin, up, down, and jump.