The reality is each sail is an airfoil. So air flowing over the airfoil produces lift and drag forces. The orientation of the airfoil with respect to the air flow determines the relative magnitude of the lift and drag components, and their directions. So if one looks down from above on a jib and considers the lift and drag forces, the lift force will have a component pointing ahead and a component pointing perpendicular to the airfoil, which is the force causing the boat to heel. Then the drag force points aft and tends to slow the boat. The spinnaker is just another airfoil. So on a reach the spinnaker exhibits force vectors similar, but different relative magnitude, to the jib. Now with the wind astern, the spinnaker lift force pushes downwind on the spinnaker, and the drag force points astern. The spinnaker speed through the air is less than the airspeed dead downwind, so there is drag. It's pretty much the same whether the sail is a jib or a spinnaker, downwind or upwind. Bernoulli's equation, lift and drag. All the same.