What could you do if you were caught green-handed?
There are dozens of techniques to move an object around the hand and vanish it. There's the 'French Drop.' It allows you to fake take something and retain it in the hand still. For a long, skinny object like a joint, you can do this easily.
If we are addressing a bag of weed, specifically, it could be handled in various ways using [an approach to communication] called NLP—Neuro-Linguistic Programing—meaning a skilled magician can apply the best strategy to either charm, entertain, influence, or manipulate the situation.