Modified from Average Cap [aamonkey] and Top Cap [aamonkey].
The top cap (red line) can be seen as an absolute market top that may never be significantly exceeded and so is a good indicator to sell a position, or enter a long-term short-sell.
The mid cap (yellow line) can be used as an indicator for market sentiment. Above the mid cap, the market is in a bullish environment, and below it, the market is in a bearish environment.
The bottom cap (blue line) can be seen as an absolute market bottom that may never be fallen short of, and so is a good indicator to enter a long-term buying opportunity, or exit a long-term short-sell.