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so you mean that you can predict what will happen in a few days or something.??
i think that the best way is to take an amount of money that you can say them bye-bye easily if something will wrong happen and to start trading - but all the time all the little wave should be reacted. not to wait weeks with the coins in the hand but to route them all the time :-))

when they all crash at the same time, there really is no point moving them about imo. If you have spare fiat to invest that you can afford to lose if it all goes wrong, then I agree this is the perfect time to invest :)

but when they are in the low position - how do you know that finally they will go back to the top again ! ?

You don't 100% know, that is the rollercoaster of crypto lol Only invest in something you believe in and only if you can afford to lose it all! :)