The quiet Boston firm’s penchant for secrecy hardly makes it an outlier in the hedge-fund world. But its unusual practices do: Adage only collects its 20% performance fee if it beats the S&P 500 including dividends (a so-called hurdle)—and it refunds up to half the charge if it fails to keep pace in the subsequent year. Adage is perhaps more suitable to that sort of practice because Messrs. Atchinson and Gross at all points keep the portfolio fully “net-long,” with a heavy emphasis on long bets over shorts, and keep 100% exposure to the S&P’s rise and fall. Most hedge...