"But you can let off your gas and not hit the break. Hitting the break is an instantaneous warning signal to people behind you, causing something of a reflex for them to also hit the brake."

...As long as you're careful about doing this, of course. If the people behind you aren't paying enough attention to notice a slow decrease in speed (and can't see the reason you're slowing down), they'll be hitting the brakes hard to slow down when they're suddenly too close.

In hilly city driving (hi, SF), I will actually intentionally use my brakes to signal to the people behind me I'm trying to slow down, because there's often not enough space to notice when your line of site effectively stops at the next intersection's brief period of flat road. And yeah, this behavior was inspired by seeing someone rear-ended right behind me.