When taking photographs, most people take them in “landscape” orientation—the picture is wider than it is tall. There are some shots, though, that require “portrait” mode, which are taller than they are wide. For example, a shot of a waterfall or a towering pine tree may be much more impressive in portrait mode than in landscape mode.
iPhoto obviously doesn’t care how you shoot your images. But it does know which are which, and varies how certain tools work based on the orientation of the image. For instance, when you’re editing a photo and using the Crop tool. If you’re editing a portrait mode photo, and choose a fixed ratio (such as 4x3 or 5x7) from the Constrain menu, then the Crop tool will crop in portrait orientation as well. If you do the same with a landscape photo, the Crop tool will work in a landscape orientation.
However, what if you have a landscape photo you’d like to crop to a fixed ratio vertically, or a portrait photo you’d like to crop to a fixed ratio horizontally? This doesn’t happen often, but when it does, it can be a bit frustrating trying to select the proper crop area. It turns out that there are two ways to do this. First, you can click the Constrain pop-up menu again after picking your fixed ratio. Scroll down to the end, and you’ll see Constrain as Landscape (or Portrait, depending). Choose this option, and the crop area on your portrait mode photo will switch to horizontal (and vice-versa on a landscape mode photo). That’s the hard way, though.
The second, and easier, way is to hold down the Option key prior to dragging out your constained region. When you do, iPhoto will select the opoosite orientation from that of your photo. Note that if you have previously activated the Crop as Landscape (or Portrait) option from the pop-up menu, then holding down the Option key will reverse that behavior (so portrait mode photos would get a vertical crop).
I find it much simpler to use the Option key, as opposed to the pop-up menu setting, for those rare cirumstances when I need to apply the “wrong” orientation crop to an image.