The exhaust brake has sensors that read a lot of things. The main thing it reads is deceleration. In TH mode it is the most aggressive but it will very it's power by the deceleration factor. If you are going up a steep enough grade that when you let off the pedal the truck loses speed fairly quickly it will not come on. If you are coasting to a light on level ground it will only come on enough to slow you at the set deceleration rate. This rate is different in TH mode and not in TH mode. My truck will apply the exhaust brake and also gear down with the TH mode off but just not as aggressively. When in the Smart Brake mode it senses the speed you want to go by when you let off and touch you break pedal. If you start over a hill and touch the pedal (hard enough to turn the brake lights on) at 40 , then the exhaust brake will do what it needs to do to keep you at 40 or to the best of it's ability. I have seen 150 HP on my exhaust break gauge but that I believe is max it will put out.
I don't believe there is a minimum speed the brake will work but there is a minimum RPM which is around 1000 RPM. My truck with 3.42 gears will work at 30 or so it just depends on the gear the transmission is in when you let off the go pedal.