All you see in the left view, titled 'Source', will be enlarged. This can be the complete original image or ( after cropping ) a region of it. Additionally you see a gray square in the middle of the view, which marks the current location of the preview.
The interior of this square - scaled by the current zoom factor - is shown in the right view titled 'Preview'. You find it in the same tabbed view as this help, click onto Preview above. This view shows only a part of the result. It is only for giving you an impression of the zoom and for testing enlarging parameters.
When you move the mouse into the 'Preview' window and hold down the left mouse button, you can drag the contents around. You can also change the preview by grabbing the gray frame in the left view ( with left mouse button down ) - if you move it around, the contents in the right view will change accordingly.
By holding down the left mouse button elsewhere in the left 'Source' view, you begin selecting a cropping rectangle. Drag the mouse from top-left to bottom-right of the part you want to enlarge ( with button down ), after releasing the button the 'Source' view will switch to the selected part, pressing 'Enlarge & Save' will now enlarge only this part of the source. If you want to undo your selection, press 'Undo Cropping'.