Reshitting is the act of taking your partner's stool, putting it into a freezer until it is hard.
Then inserting it into your own anus. Then when it warms, you push it out, thus "reshitting" what your partner has created for you to experience.
Shok wanted Sheri to experience the large stool he produced. After he froze it, Sheri inserted it into her anus so she could enjoy it also through reshitting.