In the final version of the game, Crono gets an assisted-jump, Marle tosses ice magic to Crono's sword at the height of his jump, and then Crono comes down with flames somehow on his sword.
In a beta version of the game, and on the box art, Marle tosses
fire magic to Crono's sword. In the beta screenshot, Crono and Marle are both clearly in the first half of Crono's jump.
At some point in development, Marle was a fire magic user, and then she was changed to an ice magic user, and the flames present in Arc Impulse (which is the triple-tech referenced in both the box art and the beta screenshot, one of which was amusingly based on the other) are a holdover that was overlooked.