DQ...I think Gord May is playing fast and loose with the term since he is the one that wrote what you copied. It is nevertheless descriptive of where the line is affixed to the boat.
Both websters and wiki define spring lines as to docks only and wiki has a nice diagram here:Mooring (watercraft - Wikipedia, the free encyclopedia
Can't find any source other than Gord with this use.
No big deal but would be interesting to know if ANYONE can find a sailing related source like Bowditch (don't have one handy!) which mentions springs in anything other than a docking use as I do like to be correct in my usage.