Commensurate and Incommensurate Spin Fluctuations in YBa_2Cu_3O_{6+y}

We present an interpretation of the recent neutron data on the commensurate and incommensurate spin fluctuations found in YBa_2d$Cu_3O_{6+y} based on a special configuration of the electronic dispersion and intervention from the d_{x^2-y^2}-wave superconducting phase. The observed switch over between the commensurate and incommensurate fluctuation spectra at the change of frequency or temperature is naturally accounted within this scenario.