This tool is used to position tracks by moving their control points. Light blue means end point and white control point to define the direction.
If a track is selected when the tool is activated this track will be modified. Otherwise you can click a track to select it.
Move the track end onto another track to connect to it.
Hold down shift for straight track.
Right click to change curve algorithm (all curve algorithms doesn't work for all control points).
Press enter or select another tool to save changes. Press esc to go back without saving.