This indicator is derived from the weekly net advances, the number of advancing issues less the number of declining issues. By applying 2 exponential moving averages (short and longer one) of net advances this is a perfect momentum as well as breadth indicator.
WSC used a slightly different formula to calculate the McClellan Oscillator.
Bullish: McClellan Short closes above McClellan Long
Bearish: McClellan Short closes below McClellan Long
Watch out for divergences between the market and that indicator