It\'s a bit like asking the best way to pound a nail with screwdriver. The answer is use a hammer instead.
The reason a backlight scenario is used vs. sun as key light is the squinting problem — it\'s difficult to get light into the eyes without blinding the subject. There\'s no good way I know around the same problem with a reflector so all things considered its better to use flash unless you have a phalanx of assistants to hoist and hold down scrims to diffuse the sunlight.
Conceptually flash outdoors is really just like a three light studio lighting scenario except the sun, not a flash, is acting as hair light. Two flashes for key and fill needed in front to fully control the modeling and lighting ratio. One flash will allow control of fill or modeling depending on where is placed but not both..
Use the Natural Light To Best Advantage...
The first step before adding flash should be using the skylight to model the face. Look at the eye sockets. If darker than the cheekbones the brow is shading them. Have the subject look up until the light reaches the eyes. Then turn the face just as you would by a window to highlight the front plane of the face and put the side in shadow...
After raising the face to the light you\'ll need a 3-4 step ladder to raise the POV of the camera to match the angle of the upturned face. If you try this suggestion you\'d see the face looks the same as when looking at it from ground level eye-to-eye, but it now has better light in the eyes and better modeling. Shooting down will however foreshorten the body. Minimize that by shooting from further away with a longer lens than you would indoors.
Now add flash to the face over the skylight. Not one, two for key modeling and shadow fill just as indoors.
You can put flash anywhere relative to the face but I find the best approach is to use the dominant direction of skylight to model the face just as I learned to by window light then add the off camera flash at the same angle the natural light is hitting the face. It creates a dual key light effect - the big sky softbox behind the smaller more direct flash.
The technical point to grasp here is that any flash placed off axis in front of the face acts as key light not shadowless fill. Off axis lights cast shadows the camera sees an those shadows wind up as dark voids on the face in the photo.
The parts of the face the off axis \"key\" flash does not hit outdoors remain lit only by the skylight will be rendered very dark by the camera. So just like shooting indoors to control the tone of the shadows a second flash near the camera axis where it is shadowless as possible is needed as fill.
So conceptually apart from the fact the ambient light is brighter and used to help model the face increasing the exposure on the shaded flash is really the same as lighting one indoors. Fill is needed to lift the shadows and the key light overlaps the fill to create the highlights.
So shooting outdoors with speedlights use these scenarios...
Full face / butterfly: Nose aimed to sky so shadow falls down. Flash on bracket acts as second \"key\" light on top of the \"butterfly\" sky modeling on the face. Slave is placed below me in front of the ladder about chin level with subject faces as fill overlapping the wraparound fill effect of the skylight.
Oblique / Short lit: Face is angled 45° to the skylight to create \"short\" light modeling on the face. Off camera flash moved 45° from the nose and above head so it hits at the same angle for double key effect. Flash on the bracket now becomes the fill source. It can be kept on the bracket or removed and held below closer to chin level.
In both scenarios the tone of the shadows is controlled as light as I want them to as dark as the skylight alone renders them with fill power. Key light power is determined empirically by raising power until the skin highlights or white highlights are seen clipping then reducing power by 1/3 to 2/3 stop so they retain detail.
Logistically its simple because there is only a single stand to wrangle, squinting isn\'t a problem, and it is possible to both keep detail in the sun lit hair and lift the front side to match perceptually (about 1/2 stop darker) with a full range of tone that looks normal with natural looking modeling. It complements the skylight rather than trying to overpower the sun.
Try it sometime, try everything else and use what works best for you.
