If a person is confident in their knowledge, confidence and control of the situation, then provoke a very difficult assignment. Therefore, to express doubts about the competence sobesednika- it means to shake his confidence in their own abilities and doubting man easily manipulated. This is very popular psychological trick in communication and protection from her only one - remain unconvinced of their own knowledge, abilities and capabilities.
This is not all psychological tricks that are used when communicating to achieve the desired result of conversation. Arsenal skilled negotiators, speakers and others, designed to convince, has dozens of such psychological tricks and if you do not want to communicate with you manipulated, it is necessary to learn how to recognize them.