This screen finds stocks whose MACD has just crossed below zero. The MACD(12,26,9), created by Gerald Appel, is the difference between the 12-day and 26-day exponential moving averages; its 9-day EMA is the signal line, and the zero line marks where the two averages are equal. A move below zero means the 12-day EMA has fallen below the 26-day EMA, confirming that the trend has turned down.
