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You have to pass current through the LED side of the opto-coupler for it to work. When it conducts, the first transistor turns off, this turns the second transistor on and fires the SCR to make the light turn on. SCRs stay conducting until the current through them is removed so once triggered the light will stay on. The switch shorts the SCR out so no current flows through it and it stops conducting. As long as the current through the opto-coupler LED has been removed so it doesn't immediately re-trigger, the switch will make the light turn off.
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