It is important that the mirror is adjusted correct to give as good review as possible. The size of your head, distance between the eyes and riding position on the bike will require different adjustments. When being adjusted the mirror will stay in that position. Bear in mind that the mirror is a tradeoff between getting a good rearview and at the same time not disturb your forward view.
The video below gives you a good idea of both how to adjust and use Trieye. Most people need 2 - 3 rides to get used to the mirror and find the optimal adjustment.