Conclusion: Buy ETF Indice on open market and sell on close when stdeviation of previous returns is high don't look to be a good idea, because can lead to either very high positive variation or very high negative one. In other words, seems not to exist a "Linear Momentum" in short timeframe, but yes something similar to "Angular Momentum". When market is turbulent, looks like it will continue to be turbulent for D+1, with high bullish or bearish "marubozus". In that case, one-dimensional linear operations (buy or sell) seem to make less sense than two-dimensional operations (volatility options), such as Iron Condors and Butterflies strategies.