This is a great question as there are several ways in which a butt implant can be placed for your augmentation. The options that currently exist are above the muscle, subfacial, intramuscular and submuscular placement of the implants. For the most part they each have their advantages and disadvantages.
Placing the implant completely above the muscle is done less commonly as the implant is more likely to shift and it releases some of the natural attachments of the buttock. Submuscular was popular in the past as it created a buffer for the implant in an attempt to give patients a more natural appearance and feel to their butt augmentation. It has loss popularity as it can put people at risk for complications if the implant shifts. Because of the issues with placing the implant above and below the muscle the subfacial and intramuscular implant placement techniques have developed and have continued to increase in popularity.
The subfacial technique utilizes the strong covering of the muscle. The fascia is what covers the muscle and is the strongest part of the muscle. The subfacial technique for butt implant placement avoids the problems that can be associated with placement of the implant below the muscle. It also is a much more secure pocket than can be achieved when placing the implant in the typical plane above the muscle. This strong covering will decrease the risk of the implant shifting over time. The fascia is strong and will help soften the edges of the implant.
The intramuscular technique like the subfacial method was developed to help with the issues associated with other methods of butt implant placement. The idea is that it avoids the nerves and other structures that are found in the submuscular position, while giving some buffer to the implant by splitting the muscle. The issue with the intra-muscular placement is that the muscle is typically thinner than people expect which leaves very little muscle below and above the implant. Also unlike the subfacial technique the intra-muscular placement is not a natural plane so there is an artificial division of the muscle. This can damage the muscle and lead to more pain post-operative. If the nerves in the muscle are damaged the muscle can lose some of its strength overtime.
There is no one absolute way to perform a butt augmentation with implants. It is important to discuss the procedure with you board certified plastic surgeon that performs a high volume of butt augmentations.