When descending during a single-leg squat, sit back as if sitting down on a chair and maintain a neutral spine (ie, flat lumbar spine) throughout the motion. The torso will incline forward from the hip (not the spine). The arms may be held straight ahead for balance, and a light weight may be held for additional assistance.
Courtesy of Jon Patricios, MBBCh, MMedSci, FACSM, FFSEM (UK).