In a bull market, you expect smaller-cap stocks to rally farther. In a bear market, you expect them to decline farther. The "generals" are less volatile than the "cats & dogs".
You could have noticed something in the recent weeks leading up to last on the RUT. When the 3 other majors were pushing up hard, RUT stalled consistently at 855.
Since it is small caps, they are a more representative index for US stocks. The SPX has a lot of international exposure. So if you want to short the US stock market, you'd go more with RUT than SPX.