When Modify Crop Rectangle is clicked, an initial cropping rectangle is drawn using by clicking the mouse to set up an initial rectangle on the active movie. The borders of this rectangle can be moved by placing the cursor over them. Once an arrow symbol appears, the border can be dragged with the mouse. The movie can be played with the cropping rectangle showing to adjust the borders to include the desired image.
The movie will be cropped to this cropping rectangle when Crop/Uncrop is selected.