Final answer:
Hair straighteners are the technique used to keep curly hair smooth and straight while retaining a beautiful shape.
Step-by-step explanation:
The technique used to keep curly hair smooth and straight while retaining a beautiful shape is using hair straighteners. Hair straighteners, also known as flat irons, use heat to temporarily straighten the hair. When the hair is passed through the straightener, the heat breaks the hydrogen bonds that give the hair its natural curl, resulting in straight hair. This technique allows individuals with curly hair to achieve a smooth and straight look while maintaining a beautiful shape.