trendline is simply the result of the collective sentiment.
If you compress all the candle bars the trend lines cross.
you will get something like this.
it tells you at which point, the collective result from every trades stops going lower then reverse.
You could imagine it as a market place slowly gets more and more crowded.
there're people leaving and coming all the time. Each of them has mind of its own(like a trading bot or person who follows one's own logic to participate).
Since each bot would stick with the same algo. without switching to a completely different set of algo. its action would be somehow linear.
without sudden change of environment(bus full of people dropped off, terrorist attack etc.)
It should more or less come up with the result like the trend line.