Wikipedia defines the martial arts technique of Empty Force
as "the expression of force without making physical contact."
This technique claims to harness the power of qi, the "body's vital energy", enabling masters of the art to defend themselves against opponents without making physical contact.
However, "Some proponents of martial arts are skeptical about demonstrations of empty force and dismiss them as tricks."
Recently a Finnish empty-force master, Jukka Lampila, gave a demonstration in Barcelona. So some skeptics decided to put his claims to the test.
So basically, empty force works brilliantly as a method of self defense, as long as your attackers believe in it too. Otherwise, you're in trouble.
Here's another video of Lampila doing his stuff. This time unchallenged.