The S.E.C. on Thursday proposed banning what are known as flash orders, which use powerful computers to glimpse at investors’ orders. The practice is often associated with a controversial corner of finance called high-frequency trading, which has grown, largely hidden from view, into a potent force in the markets. … Critics say flash orders favor sophisticated, fast-moving traders at the expense of slower market participants. Using lightning-quick computers, high-frequency traders often issue and then cancel orders almost simultaneously and get an early peek at how others are trading.