A pop-pin and a method for using the pop-pin with a weight-training machine to perform mixed-weight sets of exercise are disclosed. A pop-pin having a spring loaded into an unstable equilibrium position can be inserted into a weight stack to select a weight and vertical guide bar while another conventional pin is inserted into the same weight stack and the vertical guide bar at a position above the pop-pin. A user can then operate the weight-training machine, at which point the spring on the pop-pin is dislodged from the unstable equilibrium position and presses on the weight plate into which it is inserted. When the weight stack is lowered, thus reducing the load of weight on the pop-pin, the force of the spring pressing against the weight plate ejects the pop-pin from the weight plate. At this point, the user can continue the exercise with the smaller stack.