That yellow line is the %of the GDX Miners in bullish trends, and the whole complex for Bullish % indicators can be referenced here: http://stockcharts.com/school/doku.php?id=chart_school:technical_indicators:introduction_to_mark#bullish_percent_index_bpi
Its helpful for keeping an eyeball on it.
As a general rule, going long AS Soon AS an Index goes Over-Bought will work.
After its been Over-Bought awhile, or when it's ready to drop out of orbit [extreme]
is a "buy high, sell higher " proposition, ratcheting up the risk for each time unit consumed in EXTREME status. It surely does not fit my idea of Buy Low, Sell High.
Without evidence of "RANGE SHIFT", Incremental or Quantum, I am reluctant to knowlingly Buy High intending to sell higher.
Also with Reversal Analysis, one attempts to determine when periods where extreme readings of oscillators will most likely back off, and there are indicators that assist in spotlighting those windows.
Good Luck All,