We could make a whole book about how to position hands and eyes in photos. Photographers don't just tell people how to pose; they also need to learn how to come up with new poses that brides and grooms will like.
Most of the time, having your arm more than 90 degrees gives a much more graceful and gentle posture than keeping it straight.
Why? Well, when you focus on something very difficult, it activates the parietal lobe of the brain. This brings out a VERY captivating look in the eyes.