A shunt trip breaker has 2 additional terminals on it, which connect to the trip coil. Applying voltage (usually 120V) across these terminals will instantly drop the breaker out. The ones I have used take an additional space in the panel, so squeezing one into a crowded panel may take some juggling. They also cost $$$.
Assuming that you are wanting to trip one from an ansul system, you use the normally open output contacts of the system to switch 120V to the shunt trip when the system is triggered.
If you have to shunt trip more than one circuit, you can wire the shunt trip coils in parallel, to trip all of them from one input.
