In trading on Friday, shares of the Proshares Short 20+ Year Treasury ETF (TBF) TBF entered into oversold territory, changing hands as low as $28.49 per share. We define oversold territory using the Relative Strength Index, or RSI, which is a technical analysis indicator used to measure momentum on a scale of zero to 100. A stock is considered to be oversold if the RSI reading falls below 30.In the case of Proshares Short 20+ Year Treasury, the RSI reading has hit 24.0 — by comparison, the RSI reading for the S&P 500 is currently 47.8. A bullish investor could look at TBF's 24.0 reading as a sign that the recent heavy selling is in the process of exhausting itself, and begin to look for entry point opportunities on the buy side.Looking at a chart of one year performance (below), TBF's low point in its 52 week range is $27.62 per share, with $33.37 as the 52 week high point — that compares with a last trade of $28.50. Proshares Short 20+ Year Treasury shares are currently trading off about 2.3% on the day.