Why does trendline work?

In my personal opinion and experience, trend lines never work. No matter what the time frame you are using. One of two things will happen, either you try to predict future trend lines or you try to trade existing trend lines. If you try to predict trend lines, you will get burned so many times, you losses will out weight your profits. And if you try to trade existing trend lines, by the time you are ready to get in, the trend lines either brakes or is never reach. There are some exception for long term trends but for the most part, I don't rely on trend lines. Thats just my personal opinion.

the "self-fulfilling prophecy" explanation logically makes the most sense to me. however, it gets a bit confusing thinking about how these self-fulfilling prophecies get started, especially for something relatively more complex like fib levels. before we had trading platforms that map out the levels for you, somebody must have taken a lot of time to figure out that s/r levels usually correspond to a fibonacci ratio.

...either that or the connection to the fib ratios is contrived (the s/r level is bound to be close to one of the five lines, plus it obviously doesn't work 100% of the time). either way, we now have a market where thousands of people look for the same s/r levels, so they are bound to play themselves out (sometimes...). as far as i know there isn't a convincing mathematical reason why trend lines or fib levels work, and as this thread shows, it's debatable whether or not they work in the first place.

that's my inexperienced/newbie 2 cents anyway.

You need to use trendline with other indicators such as MACD and RSI. It does not matter with timeframes you on. Some indicators are more suitable for some traders like my case if prefer to use MACD with 3 timeframes . I will look for the double tops, double bottoms, triangles BO and rectangle BOs before entering a trade.

For bigger timeframe , the bigger the pips are involved.

Do look at previous day or week chart to get a feel of the pairs. Do trendlines on the 1hr, 4hr and daily to get more visual understanding of how to use the trendline to your favour.

I think it may be that BOTS are programmed to KNOW when they have CROSSED a trendline.

it's not the trend line causing price to react but the other way around.

Usually the trendlines works in M30, H1, H4, Daily, Weekly and Monthly. Larger time frame more valid.