Buying winners and selling losers may not be the best approach although it may sound like a good piece of advice. Worse so, if following this blindly by selling whatever is dropping and buying whatever is rising.

Whatever goes up will come down, but whatever goes down may not have a chance to go up again. Many market leaders in their time has crashed into bankruptcy or never resurfaced after their share prices went down.

That said, buying winners may not always be the best course of action and selling losers would  seem to be a better idea. But as prices go down, it could be hard to ascertain that this is a loser, and not a victim of economic conditions. Selling them instead of doing DCA may end up with losses which one may regret later.

I would say, the best way is to do your own research and decide for yourself (hopefully the right decisions) whether to hold and DCA or to sell them. 

In the case of Nvidia, I decided to hold on to Nvidia and to do DCA even after the bad news on the restrictions imposed. As I'm not a day trader or someone who trades frequently, I am afraid may miss the rebound after I sell it, and instead of taking advantage of the downtrend by buying it back cheaper, I might end up with actual losses. Part of the reason I've decided to hang on is because Nvidia is reported to be the market leader in the AI chip industry and I foresee AI to be the future as technology advances. 

Based on my past trading experience, I feel that if an investor wishes to make a sure profit from his investment on the long term, the surest way is to go for low risk investing but with lower returns, such as REITs. REITs are required to distribute at least 90% of their profits as dividends, so it'll be harder to lose on a REIT investment in the long term, unless one buys a REIT with a small market cap or one denominated in foreign currencies.

Before making any investments, one should always read up and research on the company, industry as well as their future prospects to minimise any risks of losing one's hard earned money. 

Selling the losers and buying the winners (to me) is just one of the styles of investing that is supposed to increase the chances of profitability. For me, I rather stick to DCA and time in market vs timing the market.