Because of this expectation you have good posture, which makes you feel more confident. Because you are feeling good, you chat to some members of the audience before you begin, which helps you realise how friendly they are. This prior interaction also means they smile at you while you are giving your presentation, which further increases your comfort and confidence. Because of the original expectation, you give a great presentation.

Because of this expectation your brain signals that there is a threat and produces adrenalin: this makes you shake before the presentation and you avoid making eye contact with anyone in the audience. This avoidance further signals to your brain that there is a threat, and by the time you give the presentation you stutter, making your panic worse. Because of the original expectation, you do not give a good presentation.