While Mr. SGNU is correct here, you must also shift your weight to your heels using the heelbone as a fulcrum to assist you thrust backwards. Another effective strategy is to simply bend your knees and lean back as if in a limbo. This would create a higher amount of surface area on the ground to assist with friction, thus slowing or stopping your fall. This would give you an effective amount of time to react and move away from the edge.