During times of uncertainty in the stock market, billionaire Mark Cuban follows what he called the "No. 1 rule of investing": "When you don't know what to do, do nothing," he said in 2016 on the Cyber Dust app.

In 2016, the market was plunging due to a steep decline in oil prices and a sell-off in Chinese stocks, as CNBC reported in January that year.

Although the circumstances differ today, Cuban still "absolutely" follows the same rule, he told CNBC Make It on Monday, as the Dow tanked more than 2,000 points.

"If you don't fully understand the risks of an investment you are contemplating, it's OK to do nothing," Cuban said in a 2010 blog post.

In fact, making reactive moves as the market fluctuates is not a good way to invest, according to experts.

"Investing should never be about a moment in time," Liz Ann Sonders, chief investment strategist at Charles Schwab, told CNBC Make It in February. "It should always be about a process over time."

It's advice that Warren Buffett often gives too. In fact, he is an advocate of the "buy and hold" strategy.

"[D]on't watch the market closely," Buffett told CNBC's "On the Money" during 2016's market fluctuations. "The money is made in investments by investing and by owning good companies for long periods of time."

If investors "buy good companies" they're "going to do fine 10, 20, 30 years from now," he said.

Sonders also points out that market downturns are part of a normal, healthy market cycle.

"There is such a thing as garden-variety corrective phases — they don't all look like the 2008 financial crisis," Sonders told CNBC Make It.

As for Cuban, the seasoned investor (with a $4.3 billion fortune) tells CNBC Make It that he sold all his stocks (except for Amazon, Netflix and Twitter) while the market was high recently, as a way to grow his liquidity (aka, cash).

But even with stock prices down amid Monday's sell-off, he's sticking to his "when you don't know what to do, do nothing" rule. He says he hasn't bought any stocks yet because of his own uncertainty.

"[I] won't rush into [buying] anything 'til I look at a company and know that it's mis-priced or see something that seems out of whack."

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