Bzzt. Screech. Zoom.
Eren's other hand along with the ax he was holding disappeared when he used his mana-based martial arts. The next thing Geer knew, his broadsword-holding arm had been chopped off from his torso right at the shoulder joint.
Eren's disappeared hands reappeared when the attack was finished. He was seen holding Geer's chopped arm by its wrist. The hand was still holding the broadsword as if it had retained its muscle memory. Something that was about to happen.
But Eren didn't wait for Geer's hand to drop the broadsword he was holding. He spun around and threw the amputated arm at the approaching Shar, who had a confused and horrified expression on his face.
Eren had coated the hand with the Blitz Shards and Igni Shards. That way, the arm was bound to explode in a few moments before it hit the ground.
