The McClellan Oscillator
reflects the short-term strength and direction of the market. Readings above zero are bullish, readings below zero are bearish. Readings above +100 or below -100 are extreme and often signal a market turn.
The McClellan Summation Index
provides a longer-term view. In general, if the index is rising it is bullish and if it is falling, bearish. Typically, the index ranges from -1000 to +1000.
In the bottom panel are the two different EMAs of the (invisible) Advance/Decline ratio that are used to calculate the McClellan Oscillator. The red 39-day EMA is called the "5% Index" and the blue 19-day EMA is called the "10% Index." If both the 5% and 10% Indexes are positive, the market is strengthening. If they are both negative, it is weakening.