Slow Motion + Freeze Action....Together?
Often we're taught that we have to have our portrait subjects 100% sharp, and admittedly as a photographer, I tend to strive for either great overall sharpness (unless I'm going for a complete abstract motion blur image, like a person walking down the subject and appearing somewhat "ghosted").
However, there are times when you can not only add a cool creative effect to your portraits but also convey a sense of movement or environmental condition by combining sharpness and motion in the same image.
In this portrait of powwow dancer Dougie Rain, I set my shutter speed to a somewhat slower shutter speed (1/60-seconds) so that I would capture movement in his feathers, which were being blown around quite strongly but some pretty heavy duty wind gusts. However, Dougie himself is sharp because (A) he's not moving and (B) my flash, which is directly primarily towards his face, helps "freeze" him in place, even if he's ever-so-slightly moving with the 1/60-sec shutter speed.
There's no magic formula to achieving this effect, and your exact shutter speed depends on how fast the motion component of your image is (i.e. 1/60-second might show motion blur with something moving really fast but not with something that's moving only slightly), and you may run into limitations with how slow you set your shutter speed to capture motion depending on how much ambient light is in your scene. On a bright sunny day, for example, trying to set your shutter speed for 1/10th of a second may be impossible unless you're using dark ND filters.
Depending on how slow your shutter speed is, too, you may find yourself in tripod range to keep your overall image sharp.
I use this technique quite a bit, because I love the dynamic feeling it gives.