Subtle changes to the shape and structure of lips can have a strong influence on feminization. The distance between the base of the nose and the top of the upper lip tends to be longer in males than in females and the upper lip is longer; when a female mouth is open and relaxed, the upper incisors are often exposed by a few millimeters.
An incision is usually made just under the base of the nose and a section of skin is removed. When the gap is closed it has the effect of lifting the top lip, placing it in a more feminine position and often exposing a little of the upper incisors. The surgeon can also use a lip lift to roll the top lip out a little, making it appear fuller.
Females often have fuller lips than males, so lip filling is often used in feminization. Injectable fillers are low-risk but tend to be absorbed after six months or so, and many implants have higher complication rates like infection or rejection. Use of fat harvested from the person can result in lumps and doesn't last long. The longest lasting and least risky results appear to arise from use of acellular dermis products.