When viewing images in the Grid View of the Library module in Adobe Lightroom, there is a shortcut to view the image in Loupe view and return to the Grid view when done. Sean McCormick wrote a blog on this shortcut and how to use it.
When you are in the Library Module and using the Grid View, there is a Navigator panel on the left column. This Navigator panel shows a preview of the image selected in the Grid view. Move the mouse over the picture and hold the left mouse button down.
As long as the mouse button is held down, the image is shown in the Loupe view. When the mouse button is released, the Loupe view is closed and the Grid view shown again.
This can be used for a quick look at a picture in a much larger size and is especially useful in the early sorting and rating stages.
See Sean’s article here.