It is achieved by metering from the sky behind the subject using spot metering in manual mode.
Then ignoring what the meter says and not changing the settings but using a flash dialled down to take the shot of the subject.
The small bit of flash then fills in the subject that would have otherwise been rendered dark if the sky was correctly exposed.
What are all these ducks beaks laughing at? :)
I would like to have taken these shots of Lewis from straight on, but behind were trees so to get just sky to make it work, I had to take it from below!
Had I metered from the face, the sky would've been white.
Its a great way to get a nice blue sky.
Give it a try, its great fun and the results are inspiring.
(Yes, I know I should've cloned out the telephone wires - it was a spur of the moment thing!!)