This indicator is derived from the weekly net volume, the number of advancing volume less the number of declining volume. By applying 2 exponential moving averages (short and longer one) of net volume this is a perfect momentum as well as breadth indicator. The Modified McClellan Volume Oscillator is a momentum/breadth indicator that works similar to MACD.
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