The full mammoth form granted by this fruit gives the user a massive frame, significantly increasing Jack's already abnormal size. Jack can level buildings with a single swing of his trunk and leave imprints of his feet on the ground just by stepping on it.
However, the user is extremely heavy and slow in this form, leaving them unable to dodge high-speed attacks. Otherwise, the user is affected by the standard Devil Fruit weaknesses, though Jack, being a fish-man, can remain conscious and breath underwater for indefinite time, although remaining unable to move in the meanwhile.
Jack has used this fruit to improve his combat capabilities; he can cause massive destruction with a single attack and bulldoze nearly anything that gets in his way by charging forward. So far, his only displayed attack has been swinging his trunk to damage his opponents and surroundings.