There are some important aspects to how breast implants are placed when under the muscle. Most implants are placed under for better coverage, primarily in the upper part of the breast. The pectoralis muscle is shaped like a Japanese fan, with the "handle" attaching to the upper arm and the edges attaching to the breast bone and the rib cage. The part that attaches to the ribs is higher than where the bottom of the breast is, so it is usually detached in order to let the implant sit lower where it is centered under the breast. The problem with this technique, often called "dual-plane" because part of the implant is under and part covered only by the breast, is that the part of the muscle that used to attach to the rib cage now attaches to the implant capsule where it can pull and cause what are called "animation deformities." The split muscle technique avoids this by covering the upper part of the implant but leaving the lower half behind it. That way you get coverage where it is needed but avoid animation problems. (Total under muscle coverage isn't practical for augmentation for a number of reasons.)