When both MT2 and Gate are positive
In this junction P1N1 and P2N2 are forward biased whereas junction N1P2 is reverse biased. The gate current flows through P2N2. As the gate current is increased junction N1P2 breaks down. It means gate current has injected increased sufficient charge into P2 layer triac starts conduction through P1N1P2N2 layers .
figure When both MT2 and Gate are positive