If a Left+Drag is continued beyond the radius of a curved track (Section 2.1.2) (not a Circle) a tangent to the curve will be created.
Creation of the tangent begins when the distance from the curved track center to the cursor is greater than 110% of the radius. If the cursor is moved within the radius of the curve then the current command exits and the Changing Track Length (Section 2.2.7.1) command is executed instead.
If Easements (Section 2.10.4) are enabled then an easement curve will be generated (if required) to connect the old and new tracks.
For detail about;