Well, definitely not a hot shot ski instructor, but I'm glad others have brought up the transverse abdominal stuff. Its not really a question of tightening the abdominal muscle, its more of a sensation of the abdominals lifting and lengthening.
As far the its relationship to leg action, the transverse abdominal muscle should activate when weight is placed on the leg. If it does not, the pelvis will be unstable, which will effect the SI joint, causing a slight twisting action which changes the alignment of the femur so that it torques in the direction of the knee.
A good image is to think of pulling up in the front and down in the back. Your lower back muscles will have more support, and you trapezius will be more relaxed.