In this note, we consider the opposition of two mechanisms for blocking-onset in a somewhat different way from Dong and Colucci (2007). We assume a region of localized confluence and diffluence, which is caused by a disturbance. The disturbance consists of a localized potential vorticity pair in a basic westerly flow. We introduce a unit vector P, which lies in the direction from negative to positive potential vorticity center of the pair. The westerly flow is accelerated (or decelerated) by two forcing terms, advection term and deformation term. The former is the advection of disturbance potential vorticity gradient vector by the westerly basic flow. The latter is the deformation of planetary vorticity gradient vector by the disturbance deformation matrix. In which direction these two terms accelerate or decelerate the westerly flow, it is dependent on the orientation of P. In any case, the advection and deformation terms are anti-parallel to each other and tend to cancel each other. The advection term dominates over the deformation term for a disturbance of relatively short scale. On the other hand, the deformation term dominates over the advection term for a disturbance of relatively long scale.