The only "SCR" I know is a silicon-controlled rectifier -- basically a normally-closed switch that is opened at a theshold voltage and cannot be closed again until the current through it stops. They're commonly used for protection circuits on ICs -- if you oversupply the part, the SCR opens and conducts large currents, protecting the more sensitive circuitry for a while (until the heat dissipation blows the chip up anyway).
I don't think this is what djreidt means, though..
- Warren