Path
The STS mode guides the putter on a straight back and straight through path. Simple, just like a pendulum.
Face Angle
The STS mode keeps the putter face square to the target line throughout the stroke. Face angle at impact is the greatest determining factor in getting a putt started on the target line. Keeping the face square to the target line throughout the stroke drastically improves the chances it will be square at impact.
Pendulum Arc
The STS mode swings the putter in a true pendulum arc, which minimizes hand action and stroke breakdowns.