Volatility is a good thing—at least for those whose livelihoods at least partially depend on market activity.

E*Trade and Charles Schwab on Tuesday revealed that their retail investing clients set new trading records on Aug. 24, the day the Dow Jones Industrial Average briefly dropped by a record 1,000 points soon after the opening bell. That momentary plunge marked the lowest point for the Dow in a year-and-a-half, and the blue-chip index hasn't been within 280 points of that day's low since.