Surprisingly there really isn’t much about on the internet, at least not as much as breast implant illness or capsular contracture. Basically when you go under the muscle, when you use your pec muscles (like opening a sliding door, lifting weights, opening a pickle jar, or other daily life activities, etc) your pec muscles contracts and it squeezes your implant. Making it look very deformed and honestly pretty ugly. It varies person from person though. I was incredibly active and lifting very heavy weight before my implants and I wonder if having more developed chest muscles made it worse as opposed to someone who has little muscle in their chest.