Horizontal vibrations are one of the two possible sets of movements that occur in a vibrating plate machine. On one hand, you have the vertical and on the other, there is the horizontal. Combine these two movements and you get a fully-fledged set of movements that put your body under action.
The best thing about having the entire set of movements at your disposal is that you will always get the most out of it, no matter how you intend to utilize it. Slimming, toning, a relaxing exercise, your entire body will be set in motion by the machine.
All you have to do is try to get the most out of it and try not to overstate it. Exercising the vertical movement along with the other ones is very good for your health but should under no circumstances be the only way to use it. Less strenuous exercising can be just as effective as long as you take the time to do it as it is meant to be done.