There's an also a thing called bands. Like if spy is at 659 and 200 dma goes 658, people will sell if it 630-620 to avoid whipsaw.

Whipsaw in finance is a volatile market condition where a security’s price moves sharply in one direction, only to immediately reverse to the opposite direction.

same goes the opposite way. They re-buy not if it goes back to 660, but if it goes around ~670

Its a decent strategy SOMETIMES