Shackles are used in lifting for various reasons. They attach to lifting points that chain hooks can’t attach to. The shackle attaches to the load via the lifting points, then the slings or chain hooks attach to them.
It allows for slings to attach directly to the shackle, which isn’t always possible on lifting points. This allows for more freedom of movement on loads that require it.
The pin gets taken out, and the shackle gets fed through the hole of the lifting point. Then the sling, chain carabiner, or whatever accessory being used hooks inside the shackle before the pin gets put back and tightened.
Shackles also get used to connect lifting accessories to each other for more reach. Like lengths of wire bond that attach to each other, or similarly, slings.
Bow shackle
Bow shackles have a rounded shape, which allows for more than one accessory to be attached to them at one time. The pressure of the lift is able to pull slightly on the sides of the shackle. This isn’t permitted with D-shackles, where the pressure has to pull directly on the rounded part of the shackle at the end.
D-Shackle
The points of pressure on a D-shackle during the lift should be at either end of the shackle in a straight line. On the pin and the rounded end. So these are not suitable where multiple accessories are to go in the shackle, or where slings or chains are angled to lift.
With any type of shackle, if it turns during the lift so that the pressure is on the sides, the loaded should be landed and reattempted.