Part 4 Chemistry summary:

Sn1 Reactions: Step 1.5-Rearrangement

In this part, Cathy and Danielle switch roles, rearranging the structure of the band. A rearrangement occurs when an atom or group attached to a beta carbon (a carbon adjacent to the carbocation) shifts to become attached to the carbocation; in this case, consider instruments as groups attached to the beta carbons. When the band rearranges, instruments change owners, and the spotlight (and charge) goes onto Danielle, who becomes the lead singer, stabilizing Cathy. Danielle has essentially become the carbocation of the molecule.

Extended Summary here.