ok phillips curve says that there is a short term trade off between unemployment and inflation, which means to achieve a lower unemployment rate, you have to have higher inflation. The NAIRU is basically the unemployment rate which does not threaten inflation.
However, due to effective govt policies targeting low inflation as well as micro reform and structural change, we are able to achieve a lower rate of unemployment without threatening an increase in inflation. ie, we can achieve a lower une rate without inflation spiraling upwards (which is the trend shown in the phillips curve)
this doesn't affect demand and supply as such, but it affects what the NAIRU is.....so right now, we can achieve a lower NAIRU without inflation sprialling upwards