Squat wedges or Slant boards are a great way to side-step potential knee issues as the angle of the wedge reduces undue strain on the knees and better targets the major leg muscles that are supposed to be recruited during the squatting motion.
While squat wedges won’t completely fix and correct your squatting technique, for lifters who lean forward too much when squatting or who fold up like a lawn chair when squatting (in other words, excessive forward trunk flexion) will benefit the most from using squat wedges/slant board.
Squat wedges work in the same way as thick-soled squat shoes. The elevated heel or wedge helps you to squat lower and deeper by increasing the range of motion in your ankle.
Although I only have anecdotal and personal experience to use to support this, I find that squat wedges also help your spine remain a more neutral position when squatting.
Squat wedges help you squat lower, squat with a straighter back, and squat with less strain on your knees.