"oh my god, I can’t tell the difference between the activity from the outside and what I consider to be a voluntary movement!" - Rodolfo Llinás is a giant in neuroscience, having published over 500 peer reviewed scientific articles.
In short: If unknown to you a person stimulates your brain in such a way as to make your leg move, you will feel that it was your own decision to make your leg move at just that time!
According to Llinás this means that the feeling of having willed an action simply occurs when your brain is able to anticipate what will happen next, e.g. that your leg will move.
Explanation of the experiment is found 36:00 in this interview with Rodolfo Llinás:
Does this experiment show that free will is an illusion? What do you think?
Be careful what you answer though! Another experiment has shown that “beliefs about free will can change brain processes related to a very basic motor level.” ;)