We develop a master equation approach to study the backaction of quantum point contact (QPC) on a double quantum dot (DQD) at zero bias voltage. We reveal why electrons can pass through the zero-bias DQD only when the bias voltage across the QPC exceeds a threshold value determined by the eigenstate energy difference of the DQD. This derived excitation condition agrees well with experiments on QPC-induced inelastic electron tunneling through a DQD [S. Gustavsson et al., Phys. Rev. Lett. 99, 206804(2007)]. Moreover, we propose a new scheme to generate a pure spin current by the QPC in the absence of a charge current.