Which hairstyle technique incorporates twists and loops to create a decorative effect?

The technique that incorporates twists and loops to create a decorative effect is known as shells. This hairstyle often involves wrapping sections of hair into spiral shapes, which can form elegant, ocean-inspired structures. Shells are typically used in formal styles and can add texture and dimension to the hair, enhancing the overall aesthetic.

The other techniques, while they may contribute to various hairstyles, do not primarily focus on the decorative twisting and looping effect that shells provide. Flat iron techniques are mainly used for straightening or curling hair, rolls usually refer to large, cylindrical shapes that add volume rather than intricate designs, and updos encompass a range of styles that may or may not involve detailed loop techniques but are generally focused on securing the hair away from the face and neck. This is why "shells" stands out as the right choice for this particular question.