Which technique helps achieve a more voluminous style?

Backcombing or teasing is a technique specifically designed to create volume in the hair. This method involves using a comb to gently push the hair at the roots towards the scalp, resulting in a fuller, lifted appearance. It increases the texture and body of the hair, making it look thicker and more voluminous.

The other techniques listed do not inherently add volume. Straightening and flat ironing typically smooth the hair and make it appear sleeker and shinier, which can often reduce volume. Layering can create the illusion of movement and texture in the hair but doesn't provide the same immediate volume boost that backcombing does. Thus, the technique of backcombing or teasing stands out as the most effective for achieving a voluminous style.