It's not less than a millisecond, I think the minimum on-time is 15 milliseconds (you can go lower but you get disproportionate amounts of error).
It's an electrically-driven valve, a skinny tube with an injector at the end, which is embedded in the catalyst. When the fuel is released, it squirts out the injector and onto the catalyst. This causes it to decompose into steam (think putting hydrogen peroxide on a cut in your skin, but vastly more energetic). The steam is then accelerated out a nozzle, creating thrust.
As far as rocket engines go, it's not very efficient, and typically you have to heat the catalyst bed with a heater to keep the thermal shock of a firing from cracking the catalyst into dust. That, and development of "varnish" in the (tiny) injector passages - baked-on crud like a baking pan, is what causes them to wear out. One or both of those is apparently a factor in degradation of the prime attitude control set.
It's not terribly good for attitude control purposes due to the limited pulse life in the "fire once, wait 3 days, fire again" duty cycle, but it has the advantage of being very small (0.15 or so lb) and very inexpensive (I think something like $20000 a piece now, much less at the time) and has an extraordinary number of flights on it.