At many points in September, investors heard about the S&P 500 hitting a record high. But at several points in that month, the index pulled back instead of moving higher. This is called a resistance level, a technical point where buying activity weakens, leaving room for sellers to come in. You're well aware of this price movement if you’re invested in an exchange-traded fund (ETF) such as the SPDR S&P 500 ETF Trust NYSEARCA: SPY. The SPY is up just 0.29% in the 30 days ending October 1.  But if you’re inve ...