Here's the 'key' to a real rally: Expert

Trading the 'most volatile' hour

Stocks will not mount a meaningful rally until they can push through recent afternoon fades, one expert said Wednesday.

The Dow Jones industrial average shed more than 500 points in the last hour of trading Tuesday, punctuating a steep negative reversal after an earlier spike. If rallying equities avoid an afternoon drop Wednesday, it could spell an end to a string of brutal trading sessions, said Larry McDonald, head of U.S. macro strategy at Societe Generale.

"The key to a sustainable rally out of this is when that pattern starts to reverse. And maybe today's the day," he said on CNBC's "Power Lunch."

On Wednesday afternoon, major U.S. averages held more than 2 percent higher after a five-day run in which they lost about 10 percent. Selling after about 2 p.m. ET ruined a spike Tuesday, and also accentuated losses in the previous sessions, McDonald said.

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He noted the skid can partially be attributed to margin calls, when investors can be forced to sell a position or add more cash to an account to cover what they owe a brokerage. Much of that selling can come in the afternoon.

Panic also contributed to some of those late-day plunges, added Jon Najarian of, also on "Power Lunch."