First off, I agree with Rob. You don't always have to redirect the force into the ground. Doing so is usually done in practice just so people get the feel for rooting, but in actual application you don't usually do that.
To get the feel for redirecting into the ground, get into a bow stance and have a partner take your back knee and push with his bodyweight into your back heel while you stand there relaxed and concentrating on your bubbling well point just in front of your ankle.
This is called "feeding the root." Your partner is not trying to break your root, he's trying to help you get a feel for it.
From the knee, have your partner progress to your hips, still pushing towards your back heel. From the hips progress to the stomach.
After the stomach, extend your opposite hand making sure you open your shoulder joint (you probably will have to twist your hips slightly for alignment here) and have your partner push your wrist opposite your back leg, pushing the force into your back heel. Again, stay relaxed and focus on your bubbling well. Also be aware of the path the energy makes as it travels down your structure into your back heel.
This is your basic rooting exercise. After you get the hang of that one, stand however you will and have your partner lightly push you in various parts of your body. Feel a path from wherever your partner pushes you to whichever back heel feels the most appropriate to send the energy into. Take your time with this one.
The next step is to take your partner's energy into your root and then bounce it back into him at an upward angle. Play with it as you will.
And then lastly, as Rob said, take your partner's push energy and simply redirect it to the side, redirect it up or down or wherever depending on wherever that energy wants to go. The main way you do that is to move your body when you first feel your partner's push that you "create a hole" for your partner's energy to fall into.
This is actually what liu-peng or "rollback" is actually all about. The outward move called "liu" in the form is just a "training wheels" example of the actual energy involved.
Use your own experimentation with a partner to experience all the ramifications of the energy and how you might use it in an actual application.