Holding a rail, doorframe, or TRX handle for light assistance while lowering into a full single-leg squat lets you train the pistol squat pattern before you have the balance or strength to do it unassisted. The support takes just enough load off to keep the movement honest without turning it into a full-assist exercise.
Log your single leg squat with support pistol sets in 1RM.fit and watch your estimated 1RM update automatically as you get stronger.