You do not want the horse to be afraid of the whip, but you do want them to respect it. You might need to snap it near the horse and give the verbal cue for whatever gait you are wanting them to move into. Make sure they know the verbal cue in-hand first, this will make lunging much easier. If after snapping the whip behind them and they still do not move out, you may need to let it gently hit their hind end. They need to know that the whip + verbal cue means to go forward. Some horses just need to feel the lightest touch of the whip, others may need to feel it a little harder. Depends on the horse. For the most part I have not had to do more than a light touch of the whip, my horses learned the verbal cue in-hand so working on the lunge was not difficult. I believe it was for the canter cue they had to have the light touch on the hind end to fully understand what I was asking for. Keep in mind I am NOT saying to hit your horse with the whip, definitely do not want this post to come across that way! I cannot stress how important in-hand work is to work towards lunging correctly.