The biomechanics of your knee functions in such a way that there is slight rotation taking place as you're extending and bending your knee. If you have a click that could come possibly because of a slight dysfunction with that mechanism (look up on 'screw home' mechanism) especially in certain angles. perhaps the rotation is stuck at a certain angle of knee extension and upon further extension forces the knee rotate to its normal position... then, again, it requires assessment. Consult your physiotherapist.