Continuity mistake: The wagon, which is full of explosives, is on fire, so Mulan cuts Kahn (the horse) free with her sword and jumps onto Khan. She starts to ride away, while she holds the reins in her left hand and the sword with her right hand, up in the air. The wagon explodes and in the next shot, she still holds the reins in her left hand, though the sword, still in midair in her right hand, is now actually in its sheath, which is always attached to the left side of her belt. Some arrows are missing on the ground as well. Then her sheathed sword lands on the ground beside her. (00:53:45)
Answer: They were weights that he made everyone use to try to get the arrow. It's unknown how heavy they were but they must have been heavy enough to even make a strong guy like Chien-Po fall to the ground.