The difference between leading and trailing shoes is in the position of the shoe pivot. A leading shoe is pivoted at the end of the shoe facing in the opposite direction to the rotation of the drum. So on the right-hand side of a car travelling forwards, a shoe facing the front of the car would be leading if it was pivoted at the bottom.
Leading shoes provide better braking because there is a 'self servo' effect: as the friction material grips the rotating brake drum the geometry of the shoe means it tries to wedge or 'sprag' on the drum, and this pushes it into closer contact - increasing the brake force. By contrast, a trailing shoe is constantly being forced away from the drum and so offers considerably less braking force.
A leading shoe becomes a trailing shoe when travelling in reverse.
