The front limb of the dog behaves in exactly the same fashion as the human arm. Consider the diagram A. The unshaded arm is analogous to the standing position of a fox terrier - perfectly straight at the wrist. This is the correct position for a narrow terrier, because these dogs do not bring their feet under the body, but tend to move the front legs - pendulum fashion - straight ahead. (they avoid a roll by having very rigidly coupled, narrow, short bodies.)

The Lhasa, however is a very flexible, longer bodied dog, which avoids a rolling gait by "single-tracking" - bringing the feet toward the midline. What happens when the "straight pasterned" foot is brought to the midline. Diagram "A" shows that the foot no longer points in the direction of travel. but now "toes-in". During the weight-bearing portion of the stride, this puts torque on the wrist bones (pastern), and produces an abnormal gait.

Diagram "B" shows the midline foot position, with the wrist (pastern) correctly angled for the weight bearing portion of the stride.  When the foot is in the standing position (the foot directly beneath the elbow) it toes out slightly.

Thus you can see that the single tracker should not have the absolutely straight pastern of the Fox Terrier. Since the Lhasa is wider, longer bodied, and more flexibly coupled than a fox terrier, the mechanics of his front movement are significantly different. The Lhasa with a terrier front will either move wide, and roll, or it will toe in and appear to be weaving in the front. The latter problem being the more serious, because it causes abnormal stresses on the joints.