What effect does applying heat during a perm have?

Applying heat during a perm is crucial as it helps to set the curl pattern by facilitating the chemical reactions necessary for altering the hair's structure. Perming involves breaking down the hair's disulfide bonds, which are responsible for its natural shape and texture. When heat is applied, it speeds up the process of these bonds reforming around the perm rods or curlers, allowing the hair to take on the desired curls or waves more effectively.

The heat essentially activates the perm solution, ensuring that it penetrates the hair shaft and promotes the desired shape. This is why it is essential for achieving long-lasting and defined curls, making it a critical step in the perming process. The other options do not accurately describe the role of heat in perming; while other factors may influence hair texture or appearance, they do not relate specifically to the perming process itself.