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Internally a SCR is a pair of interconnected transistors (npn and pnp), the emitters are the output terminals and a base is available as the gate. When there is no gate (base) current, both transistors are "off". When gate current is inserted, this is amplified by that transistor, so there is collector current, which is the base current for the "top" transistor, which then comes on and its collector current is the base current of the bottom transistor. The device then becomes more or less a short circuit. (actually 2 X Vbe + 2 X Vsat c-e ~ 2V). Providing the current is available, it will remain in this stae until the current drops below the "holding value", which is not enough to keep both transistors on and the device switches off.
Frank.
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