How i do it?

[jrose]

Surely you could do it with a ring, spare krab, mallion etc as described by Mr Fowler?

[Perkins]

I think perhaps the picture is deceptive and the guy is only standing on one foot.

[daltontrees]

I think I see how to do this with a long endless rope sling. Tie a midline knot (a moth would do - see Arbtalk's splendid knot guide) in the middle of the sling (using both sides of the sling). This creates one footloop (the one we can't see in the photo). Then pass one end loop of the sling round the back of the stem and one round the front, put one loop through the other to make the bit we can see. Et voila!

 

Better would be a adjustable length sling for different diameters of stem. I think I see how to do this too if anyone's interested.