If the mcclellan-oscillator is the breadth equivalent of momentum, the summation index is the equivalent of position. It rises while breadth momentum is positive and falls when it is negative.
Analysts use it to characterise the market's underlying condition over weeks and months, and turns from deeply negative levels have historically coincided with the early stages of broad recoveries.
It is a slow, cumulative series and therefore useless for timing anything. It is also entirely dependent on the same advance-decline data as everything else in this family, so stacking it alongside the oscillator and the advance-decline-line gives three views of one dataset, not three confirmations.
Related: mcclellan-oscillator, advance-decline-line, market-breadth, indicator-lag, confluence