This is a suggestibility test that a hypnotist would perform to test the imagination and non-analytical abilities of a client.  During this test, a client would be asked to imagine a heavy object in one hand (such as a weight, a shopping bag, or a stack of heavy books) and a buoyant object in the other hand (such as helium filled balloons).  A highly suggestible client will have considerable movement & separation of their arms, while a very analytical client would experience no change.  Both clients can be successfully hypnotized, but require different methods of hypnosis techniques to be implemented.