Here is my current interpretation of the NASDAQ-100 (NDX) from an Elliott Wave perspective. Overviews on different Elliott Wave Patterns can be found here: http://thepatternsite.com/Elliott.html
It looks like we had a downward leading diagonal from the top on Sept. 19. A leading diagonal is an exception to the usual wave principle in that within a Wave 1, Subwave 4 can overlap Subwave 1. Highs and lows within the pattern usually follow trendlines. I think the drop shown above completes a Minor degree Wave 1.