Let's say you had enough money to buy 1 whole Bitcoin when the price was at it's lowest in Mar '19 <$3000+> and sold in May '19 <$8600>, then you would have more than doubled your initial investment.
I would rather hold for a longer period of time rather than selling out during those spikes as i am in this market for a long time to understand the market sentiments and i am not putting my credit cards amount in bitcoin to sell off fast, you need to have patience when it comes to any form of investment and the strategy you are telling will work but if the market rises after you sell off then there is no way to enter the market yet again. I do ignore small profits and i am here for the long term and hence it works all the time.