There're two types. One is called an electronic delay controller and the other is a proportional type. The electronic delay ones are the least expensive and they really suck. You step on the brakes, they begin to apply them at a set rate going from weak to strong. They do not know the difference between a panic stop and a tap to turn off the cruise control.
The proportional type (Tekonsha Prodigy being the best Ive used) has an inertia device in it that senses how hard the vehicle is trying to stop and ramps up the current to the trailer brakes proportionally. It knows when you are trying to stop right now and puts full power to the trailer brakes. I year ago October I had my family in my crewcab with my skidsteer and backhoe on the trailer and was going 80mph when some shit with a 12' long car decided to pop out into a 15' space while I was going by a semi. With a piece of shit electronic delay I would have ran him over flat. What happened was, I stomped the brakes, it sucked me down 20mph in a snap, I let go and stomped the gas and we continued on our way. The panic button on those electronic delay units are way far away from when you need reflexes to save your ass.
I do like surge brakes a lot. They also know when you want to stop right now. There is a direct-acting master cylinder in the trailer tongue. Its perfect.
I have also had hydraulic tap brakes and those are good too. I dont know if you can even get them anymore.
The best part about the Prodigy specifically is it is so easy to set up the unit. Some proportional controllers have pretty strict placement requirements. Electronic delay controllers can be placed anywhere, but the garbage can is the best place.