Late Update: This link has a movetrendline function I wrote to help with this.
Of the two end points of a trendline, the point with the earlier date and time is always considered to be the starting point; if the trendline is vertical, the point with the lower price value is considered to be the starting point.
I have always found that using these commands can give some strange results if you do not get it just right and the above statement explains it somewhat. SOMEWHAT! I came up with the two assumptions below (shown in quotes) and put that logic in my the change I have been making to a few studies. It seems to work.
1/ Therefore you should move the end point farther out in time first so it continues to be considered the end of the trend line.
2/ Also you should put the end a bit higher than the begin (a very small amout so MC does not get confused yet so you do not detect that the line is on an angle) - This was actually TJ's idea.
However, Here are the two interesting questions that come about after reading that quote from the help.
1/ Is MC actually scanning to try and figure out the beginning and ending of a trend line rather than doing a database lookup using the ID number? If so, Is that why it gets strange results if you move then beginning of the line first out past the end of the line and then it thinks the old ending which is before the new beginning is now the new beginning.
2/ The comment in the help has a conflict. What happens if the line runs down to the right. It is saying that the beginning of the line is the first time it finds but if it runs down to the right then the beginning of the line is the lowest anchor point which is second anchor point (the second one is later in time).
So I am sure someone will comment after having read the above but maybe this weekend I will write an interactive study that detects when I line on the chart has been moved manually by the user and it will put text above the two anchor points telling me what MC thinks is the beginning of the line and is the ending of the line. It will wait until the line is not active before it makes this determination. I will move it around in all sorts of strange combinations to see if I can figure this out. For sure that will be the horses mouth for the answer to these questions. I will post the code when it is done with a few comments I am sure.