When technology stocks tumbled for a fourth straight day in January, Evan Fetter, a 25-year-old in the U.S. military, saw an opportunity to swing for the fences. 

He poured $15,000 into the ProShares UltraPro QQQ, an exchange-traded product that is designed to triple the daily return of the Nasdaq-100 index, bidding for what he called a “once-in-a-lifetime gain.”