Turned on the diode means blocked current? or vise versa? That made me confusing. In here VCC and VEE is +12V, -12V. so, if Vin is 6V then upper OP amp will give +12V. and below OP amp will give voltage -12V. then D1 will be not block and D2 will block. right? so..D1 is turn on? or off? How i have to call that?
Here is a diode. If the anode is at a higher voltage than the cathode it will be forward biased and conducting or "on". The anode voltage has to be about 0.7V higher than the cathode, for a silicon diode, for it to conduct.
If the anode is at a lower voltage than the cathode it will be reverse biased, not conducting and off.