1) She stayed by Tom's side on the way to the fields because she was frightened.
2) She helps to fill Tom's bag with cotton so he won't be beaten.
"The woman suddenly turned to her work, and labored with a despatch that was perfectly astonishing to Tom. She seemed to work by magic. Before the day was through, her basket was filled, crowded down, and piled, and she had several times put largely into Tom's. Long after dusk, the whole weary train, with their baskets on their heads, defiled up to the building appropriated to the storing and weighing the cotton. Legree was there, busily conversing with the two drivers."