The Summation Index is simply a running total of the McClellan Oscillator values. Even though it is called a Summation Index, the indicator is really an oscillator that fluctuates above/below zero. As such, signals can be derived from bullish/bearish divergences, directional movement and centerline crossovers. A moving average can also be applied to identify upturns and downturns.
Indicator uses the advancing/declining stocks from NYSE by default, but you can change them to any market in the options dialog.