When a light source is struck by an object, it emits a unique frequency that can be picked up by many microscopic particles.
If we use sound as an analogy, this means that the particles that are struck by the light have their own unique frequency. These frequencies can then be picked up by the levitating particles. The problem is that when we try to use this method to levitate something, we do not have any control over how far the object will move.
Instead of the weight pressing down on an object, we need to have some kind of control over the amount of force used. This is why some folks have gotten hurt while levitating. Basically, what happens is that the force used to lift an object causes the molecules to vibrate. Vibration is not like sound.
There are a lot of theories out there about how this works, but it is simply a matter of probabilities. To get an object to vibrate, it must meet some kind of criteria. One of these criteria is that it must have a boundary. The only way to determine whether or not a given area has a boundary is to set it free in some sort of apparatus. Then, as the sound waves strike the body, it will vibrate. The amount of vibration will depend on what the object is made of.