ETS System applied historically on 1925 - 1939 chart.

The 1929 crash really occurred in 1930 and continued through 1933.  Notice how the system did not sell during the first dip down in 1929-.
the trend was still positive and the system “waited” until the right time to generate a sell signal.
Source ETS System Signals Monthly Chart on MetaStock Historical DJI Data.

*Historical Performance subject to past performance & risk disclaimers