The preview dialog has five buttons:

Below the video there is a timeline. You can click any position on the timeline to instantly go to that time of your video. It is like instantly rewinding or fast-forwarding until you reach that position in your movie. You can also drag the red cursor line on the timeline to watch as the video rewinds or fast-forwards if you would like.

There are 3 preview modes available:

Also you can press Alt + Enter to toggle full screen mode.