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When cropping, click the corner of the image and move around outside the image. It will eventually flip direction. Hard to explain, takes a little time to get used to but work effortlessly once you get it.
just adding to what Denis has said, select your crop ratio in the develop module under crop and straighten and click the lock icon to the lock position. Now you have to drag one corner so that it goes from the vertical to the landscape crop. As Denis said, it takes a bit of practice but you will soon pick it up