What is the green/red drag line in Altium?

I asked the Altium support if there is either a way do disable this feature or to get a detailed description to understand what it does.

This is loosely translated from the German response:

If you are seeing green/red lines when placing components in the PCB document, this can be interpreted as follows: It is a kind of feedback from Altium Designer regarding the length and position of the connecting lines. Red lines indicate, that there is still a better position available with respect to intersections and connection length. Should the line show green, then this is an indication that the length of the connecting lines is well matched.

I am not really satisfied with that response but I could just disable .... oh wait.

However, there currently is no way to turn off this mode during placement.

Well, at least its a great example of implementing a feature that no-one needs because its not even documented how it works :)

It turns out that this feature (Optimal Placement Vector, or OPC) is documented in the tutorial (http://techdocs.altium.com/display/ADOH/Tutorial+-+Getting+Started+with+PCB+Design) Here's what it says. As you move a component around in the workspace, a thick green or red line will be displayed, traveling from a point within the component, to a location on the board. This line is called the Optimal Placement Vector, its function is to give an indication of whether the new location is better (green) or worse (red) than the previous location.

The vector has two distinct properties: its proposed target location; and its color.

To determine the locations for each end of the vector, the feature uses the centroid of the polygonal shape defined by the locations of the end points of the connection lines. There are 2 centroids of interest, one defined by the ends of the connection lines terminating on the component you are moving (the component centroid), the second defined by the other ends of that set of connection lines (the target location centroid).

The Optimal Placement Vector is drawn between these 2 centroids, with the component end highlighted by a dot. Because it is a relative indicator, when you first click to start moving a component the vector is always drawn in green. The 2 centroids are continuously re-calculated as you move the component, because the connection lines can move from one pad to another as they are automatically re-optimized to maintain the applicable net topology for the moving component. Because of this net re-optimization, the target end of the OPV can jump around as the component is moved. If the centroids move apart and the OPV becomes longer, it may change to red. If the centroids move closer together and the OPV becomes shorter, it may change to green.

The length of the vector is not the only condition used to set the color, the color of the OPV is also affected by the overall length of the connection lines attached to the moving component. If moving the component results in the overall length of the connection lines increasing, then the OPV becomes red. Alternatively, if moving the component results in the overall length of the connection lines decreasing, then it becomes green.