Navinder Sarao hasn’t spoken publicly since he was arrested Tuesday on charges of manipulating the U.S. futures market. But in emails released by regulators, the 36-year-old resident of suburban London called himself an “old-school” trader who excelled using “intuition,” quick reflexes and a computer mouse.

Mr. Sarao was detained at his home near Heathrow Airport on U.S. charges that he profited from illegal trading strategies from 2009 to 2014, and contributed to the violent selloff in stocks that became known as the “flash...