in very basic terms a stallie, which is just a higher rpm torque converter than that found in your transmission now, acts much like a clutch. because a car needs come to a stop without stalling you need to have a device that takes up the turning force coming from the engine. on a manual this is done with a clutch, which "disengages" the transmission from the engine.
so on a auto trans the torque converter does this job. up to the converters set rpm it can easily take the turning force of the engine, meaning you need little brake pressure to keep the car stationary at an idling rpm of say 1000rpm.
once you start applying throttle while still holding the brakes you would notice that you need more break pressure to hold the car still, this is because the converter is letting more power through. having a higher rpm torque converter means you can raise the rpm higher while still being able to hold it still - this is called "stalling up", which makes for great take offs. the ideal rpm for a torque converter is a couple of hundred rpm before maximum torque, so once you reach its stall point all power will be delivered to the wheels at once.
thats about as basic as i can explain a stallie, its tricky to put in words like this
