Can anyone tell me why they do this?

I thought I'd put this picture up because I've often wondered why swans carry a foot like this sometimes when they swim. It's easier to show a picture of one doing it than trying to explain it . :-)
And in case anyone wonders, they don't swim in circles when they do this. ;-)

[Nat]

I have also wondered this.

[Susan Bell]

A mini sail?

[Hamble]

Resting? I've also noticed they can still get quite a speed up prepelled by only one foot.