I saw some traders on youtube mentioned the 400EMA for looking support and resistence, how useful it is? does it actually works?
i only saw 50 or 200 ema in most books, how to find next dynamic support or resistance if price broke 50/200 ema?
any help/guide will be appriciated…
There is no end for this.
Once it breaks 50 ema, dma etc then they will say it’s close to 100 ema etc. Then after this is broken they’ll go for for 200… 365… 400 (and also changing the time frame).
There is no best length for this, it depends on the trader and what he wants to achieve. One should not follow others, it may or may not be suitable to them.
Instead, one must examine the charts and analyse which one will suit him for his needs, expectation, time frame, mind set, capital etc all by themselves.
( This is only if you want to use technical analysis).
in my 3 year journey this is the best advice yet “There is no end for this…” lol all facts 
… you changed my mindset