[Gimp-user] Doing a select without the mouse?

[Gary Aitken <gimp dreamchaser org>]

Is it possible to do a rectangle select without the mouse?
For example, with the rectangle tool selected, one can fill in the fields in the
tool options to specify the coordinates and size of the rectangle, 
but I can't see a way to actually commit the operation;
I don't see a "do it" button at the bottom, or an option in the tool options menu. 
I'm looking to get a precisely sized selection to fill into an added space.
For example, when adding a strip at the top of an image,
I would like to grab the top 100 pixels,
flip it vertically to create a mirror image,
and paste it into the expanded canvas.
Am I missing something?

Thanks,

Gary