Purpose: most often used when spar legs are to be spread apart to form an A-frame.
Tying: Tie a clove hitch around one pole. Wrap both poles with a simple lashing. Wrap the lashing with a two or three tight frapping turns. Tie off the end with a clove hitch. Spread the poles to make shear legs
Animated Knots: http://www.animatedknots.com/lashshear/