As I said, this is a video on the basic principles of a BSS. As I also said, the goal in this essentially single leg squat is the same as any other squat variation: to get your knee to as fully into flexion as possible. You can see in the video that my forward leg’s knee isn’t fully flexed. It’s hard to impossible to get it there for me doing a BSS unless I use something to elevate my heel. Typically I use my wedges (seen in the Goblet Squat vid) to achieve full knee flexion. So, if you find yourself also coming short of flexion, and if you don’t have a wedge, get a plate or two and prop them under your heel to elevate it for this exercise to have it’s fullest effect on causing you excruciating agony, I mean, developing you massive quads.