Here you can see my client Sarah doing a Bulgarian split squat with a kettlebell loaded on the opposite arm to that of the leg that is being worked. This means the glutes have to work that bit harder to stop the knee collapsing in. In addition to this, the video on instagram will show how we were getting Sarah to pause at the bottom of the movement and ensure her hip was as low as her knee. These cues mean her glutes have to really work to control the movement - quite often i see this exercise done almost exclusively using the thigh muscle. Good coaching allows you to target the muscles you want to work in a safe manner.
My studio is based in the Bow Business Centre, E3.