With this method, you are able to force your spectator to choose, from several objects, just the one you wanted him to choose.
Let's look at a simple example. You have three objects: A, B and C. You want your spectator to pick the object A. So you tell him to select two objects and then you act according to his choice. If he picks B and C, you say you will eliminate these two and the remaining object (A) is his decision. However, if he selects A and B, you eliminate C and ask him to select one of the remaining two objects. If he selects A, that is his selected object. If he selects B, you eliminate it and the remaining (A) is his chosen object. I doesn't matter what he selects, you will always end up with A.
If you are having difficulties understanding this, here is a video explaining it:
This can also be done with larger number of objects. You may be afraid that people will figure it out because of the simplicity – they won't. As long as you don't overdo it, almost everyone will be convinced they had a free choice.