What it measures
Launch Direction is the starting line of the ball, left or right, the moment it leaves the clubface. It ignores height; that is Launch Angle. It is measured in degrees from the target line.
A positive number means the ball started right of the target and a negative number means it started left, for both right- and left-handed golfers. Zero means it started straight at the target. Where it finishes also depends on curve, which comes from Spin Axis.
Typical numbers
Tour players do not have a single average here, because they start balls on purpose to either side for draws and fades. Their spread is small, typically within a couple of degrees of their intended line. Everyday golfers usually see a wider spread, often several degrees each way.
Every degree counts: at 150 yards, one degree is about 2.6 yards of start line; at 250 yards it is about 4.4 yards, before any curve is added.
What changes it
Face Angle does most of the work. The face accounts for roughly 85 percent of start direction for a driver and about 75 percent for an iron; Club Path supplies the rest. As a rough guide, a driver with the face 2 degrees open and a path 4 degrees left starts close to 1 degree right.
Strike location also moves it with a driver. Because of face curvature, toe strikes tend to start farther right and heel strikes farther left than the swing alone would suggest.
What it changes
Launch Direction sets the start line, and Spin Axis bends the ball from there. Together they decide where the shot ends up, shown as Side and Side Total. A shot starting 3 degrees right with a draw axis may finish on target; the same start with a fade axis finishes well right.
The relationship between start line and curve is the classic way to read shots: started right and curved right points to an open face and a face open to the path; started left and curved left points the other way.
How to use it
Separate start line from curve. If the ball starts in the wrong place, work on the face first; if it starts well but curves away, look at Face to Path.
Bay drill: choose an intermediate target on the screen a few feet in front of the ball, or set an alignment stick on the floor pointing at the target. Hit ten balls and record Launch Direction. Aim to keep eight of ten within 2 degrees of your chosen start line, then repeat with a different club.