but i dontknow if they lose because im thinking that if i sell my bitcoin when it is dumping. i could invest again in a low price so i have still profit from it right ?
The problem these people face are like this;
They initially buy let's say 1BTC at $2500.
At the time the price goes down significantly, they rush to panic sell their coin at $2100.
Price goes down a little bit further, to $2000, or slightly below that level, but they think it will drop further, and thus wait as result.
Instead of going down further, the price climbs back up all the way to $2300, where again they think it will drop further, so they wait.
The price continues to increase further, and climbs back up to the $2500 level, where after that these people can stand it anymore, and buy as result.
Recap; Buy 1BTC at $2500 -> Sell 1BTC at $2100 -> Buy back 0.84BTC for the $2100 they had left from selling that initial 1BTC = 0.16BTC loss.