When the cylinder is spinning and coming down, the two main forces acting on the ball are gravity and friction. The normal force is acting on the sphere and mg is acting in the downward direction. The forces which are acting parallel to the inclined plane are friction force, fs and the component of the weight of the cylinder, mg \\ sin θ . Normal Reaction is balanced by mg\\ cosθ . fs and mg\\ sinθ are not in a line because they are R (Radius) distance apart. As a result, the torque will act, which gives rise to rotational motion i.e. rolling.