When Trace is enabled, if you use the pen tool within a certain range of an edge or an internal line (approximately 3-5 pixels), the drawn line follows the path of the edge or line. When you move the mouse outside the range, the pen tool behavior reverts to normal historical behavior and continues to draw the line.
When magnetic trace is enabled, if you draw a line over an existing edge or internal line that has points, points are automatically created in the same location as the existing points. This ensures, for example, the curve of the new line matches the curve of the existing edge or line.
If Cross is enabled with Trace, the magnetic trace continues when you cross from one piece to another.
Trace helps you draw a line to conform with existing edges or lines. It does not change the end result behavior. In other words, if you use Trace to draw an internal line, it remains an internal line. And if you use Trace to draw a piece, it remains a piece.
Trace operates while you hover and move the mouse. There is no need to click when following an existing edge or line.
Magnetic Trace Example
You select the pen tool, ensuring that on the Horizontal contextual menu both Trace and Cross are selected. You click in a piece to start drawing a line.
You hover over an edge. The pen tool automatically follows the edge as you move the mouse. There is no need to click.
As you move the mouse, the pen tool continues to draw a line conforming to the edge.
As you move your mouse and hover over an existing point, the pen tool automatically creates a point on the line you are drawing at the same location. Again, no need to click.
You move the mouse from one piece to another. Because Cross is selected, the pen tool automatically continues the line on the new piece.
On approximately the middle of the line, you click. The pen tool adds a point.
You move the mouse away from the edge of the piece. As the pen tool is outside the magnetic tool range, it now conforms to its normal, historical behavior and continues the line where you move the mouse. You click on the piece to finish drawing the line.