Buck Brannaman wants his students to know where their horses' feet are - always. As a good rider we ought to control the horse's feet. We need to know where the horse is about to place them in order to give our aids at the right time. 
Our cue needs to come in at the moment the horse lifts his foot. If the foot is already on the way to the ground, if it touches the ground or even carries the weight of the horse, we have missed the crucial moment. We ask our horse to set his foot at a time when he is not able to. We will confuse and frustrate him. But once we get in time with their feet, our horses will respond much easier and lighter, because we ask for something they have no trouble giving to us.
Pat Parelli calls it the friendly game, others speak of desensitization. Whatever you call it, the importance of it stays the same: Our horses need to learn how to deal with a whip. They need to know when it means "you better move" and when it means "stay cool and relax". The key here is our body language. The whip is supposed to be just an extension of our body, should amplify the energy we carry in our body. Unfortunately it happens that our body says one thing and the whip another. So it is vital to get our bodies in tune with the whip (or better vice versa) in order for our horse to be able to read us correctly. Once you got that your horse will feel secure around you as you are predictable. And you will be able to switch quickly from relaxation to moving the horse. 
At the beginning your horse might have trouble. Today, I worked with a gelding and I dialed him up quite a bite as he was not too responsive. Afterwards he would not turn his eyes away from me, all attentive, but he lost some of is confidence. So I took the whip and started to swing it rhythmically over his back and neck, to run it over is body. I continued until he could relax (which happened quite soon). Then I asked him to move again. Then I got back to friendly. He lost is caution and became confident again. So make sure that your horse tolerates the whip on his legs, his body - also when he is moving. He shall respect it, but he shall not fear it. Work on getting body and whip in sync, show him that he can relax even when the whip is moving.  
Image this situation: You lead your horse to the pasture and on the way you have to pass some hay bales wrapped in a tarp. Your horse stares at the tarp and spooks from time to time, especially when it's windy and the tarp is moving. You have walked your horse up there for weeks and he still is upset about these hay bales. The reason might be: You two are just surviving the situation but you are not progressing. Each time you pass, your horse goes "oh not, not that scary thing again" and each time you go "hopefully he will be fine this time." The key here is preparation. Get your horse used to plastic bags around his feet and his body when standing still and when moving. And than work with your horse around these hay bales, don't just lead him by. Get him used to it and allow him to retreat (and reapproach) if he gets in too much trouble. Warwick Schiller did an interesting video on this topic: http://www.youtube.com/watch?v=VgHZwZOhRcQ 
Just a short one on this topic I've been coming across quite often lately: Give the horse time to think and  to digest something. I often find humans to want to go on quickly after achieving a good response by their horse. Don't. Wait until your horse tells you that he has got it: He will yawn, lower his head, chew, lick his lips or loosen up. Also, it can help to not practice a task all over again, day after day. Give your horse a break of a few days and then try again. The progress you can achieve by just giving him some time can be astonishing. I've had horses that were really troubled when we tried a new maneuver. After one week of doing nothing related they were did it almost perfectly. 
Often, when we work our horses from the ground we exaggerate our body language, straighten up or move our arms more than we'd have to. In the beginning this is important as it helps our horses to read us - they know body language for sure, but they often don't know that we actually use it to communicate with them, as they often have to face contradictory signals on our part and have learned to ignore it. 
Once you have that understanding of the horse it is important to stop exaggerating. If we don't we keep on screaming at the horse with our bodies. That's far from the subtle communication we are seeking. Your raised arm will turn into a raised finger, just breathing in will tell the horse that you are up to something and he'd better listen. Horses are way more sensitive than we think they are. 
I've recently watched a dvd by Buck Brannaman who works his horses on a light lead rope. He does not close his hands around it, the rope just sits in his open palm, and his horses willingly follow this light feel. He wants the tiniest clue to mean something to the horse. It opened my eyes to what subtle communication ground work can lead. There is always a way to get better even if we think that we are already quite skilled. The quest will pay off - for us and for our horses.