Exchange-traded funds have amassed trillions of dollars by offering investors greater tax efficiency, liquidity and lower costs than mutual funds.

An ETF that just last month was dubbed the most volatile to ever hit Wall Street has already been upstaged, after the debut of a competing product that adds even more leverage.

An unprecedented amount of cash flowed into the world’s largest and oldest exchange-traded fund last week, as stocks rallied to near-record highs after the Federal Reserve indicated it could cut interest rates next year.

Brooks Friederich is a little-known figure in the world of investment advisory, even among the Wall Street cognoscenti. Yet every year, the 39-year-old — and his Berwyn, Pennsylvania-based employer Envestnet — helps steer billions of dollars into tailor-made strategies for financial advisers, part of what’s known as the model-portfolio boom.

Bill Gross is right: bond ETF activity has been frenzied in the grip of Wall Street turmoil.