While mindfulness does not directly interfere with the content of the mental activity, it does influence this content. Similar to how reality T.V. stars act slightly different when the cameras are rolling, the mind behaves differently when it is placed under the focus of conscious awareness.
In science, the term “observer effect” refers to changes that the act of observation will make on a phenomenon being observed. Similarly, in quantum mechanics, which deals with very small objects, it is not possible to observe a system without changing the system. While that is the extent of my knowledge of quantum physics, my experience has revealed that the same holds true for the observation of one's own mind-movement.
When thoughts attack, (creating fear, sadness, depression, anger, boredom, anxiety, etc.) the act of observing those thoughts from a place of detachment causes a shift in the system of thought. In order to observe these thoughts, silently place your attention on the content of your mind-movement as though you were looking in on the thoughts of someone else. As the silent voice in your head speaks on your behalf, take notice from the objective perspective of a scientist conducting an experiment. Feel the sensation of thought and the corresponding feelings in the body. Do not try to affect the outcome of the experiment. Instead, simply observe with patience and openness to whatever may arise.
The next time your thoughts go on a negative tangent, silently observe how they change course as a result of your silent and objective observation. Test this hypothesis for yourself.
Be still and know...