This is an excerpt from Sport Injury Prevention Anatomy by David Potach & Erik Meira.
Many exercises in this chapter describe jumping and landing positions. The position we advocate to maximize performance of the drills is with knees bent and aligned with the second and third toes (see figure 8.4). It is acceptable (and natural) for the knees to move past the toes; however, this movement can be considered more advanced, so when beginning the squatting, jumping, and landing exercises described in this (and other) chapters, limit that forward motion to a position level with the toes. More advanced athletes can, and should, allow the knees to move past the toes.
Note: The knees should not move excessively inward (valgus movement) or outward (varus movement). Dynamic valgus is a known risk factor for knee injuries, including ACL tear.