For a number of years, there was this mythical notion that more forward lean leads to a more aggressive, and thus more effective posture for high end skiing.
To the contrary, the right amount of forward lean supports, but does not create or enforce, an effective posture. Any more than enough is simply too much, interfering with range of motion, increasing muscle tension and fatigue, and increasing the possibility of knee injury.
If the forward lean is excessive, the athlete may not rest on their ‘bone stack’, and in the event of fatigue, must either cease activity, or try to rest on the boot tongue. Levering the boot tongue simply provides turning input to the ski, so that may not be a good idea unless the skier is standing in the lift line.
If forward lean is insufficient, the knee joint may be moved back of the nodal point of the ankle joint, a location from where it becomes difficult to influence the fore-body of the ski without large movements of the torso. This is tough on the hamstrings/lower back, and makes for some tenuous situations.