It is a small piston containing a primer. It can be applied to the thermal valve or to the thermal valve with selector.
The detonation occurs when it is fed at 12 V or 24 V; its effect pushes a piston against the fusible link, breaking it.
It has to be applied on the thermal valve. When fed at 24Vdc it releases the thermal bulb allowing the thermal valve to puncture the CO2 cartridge in order to open the cylinder.
It is reusable, it has the same function of the pyrotechnic device but has not to be discarded after use, the thermal bulb as well will not be lost (except after a real fire situation: in this case the whole system must be renewed).