Well the stock shackles alot for some movement from front to back and a rear pivot point for your leaf springs. You can also use a chevy drop shackle which will give about an inch or better of lift becuase they are longer.

 

When your chevy springs get compressed(aka they flatten) they need to become longer.

Imagine it this way. You have a ruler stick(foot long) made out of metal. You hold it in your two hands(hands on the ends) and you push the ruler so that it forms a half circle. Notice that your hands pushed in to make it into a half circle? Well, when weight is put onto your springs, which flattens them out) they become a little bit longer. The shackles are what allows this.

Aaron

 

Exactly right.

And if your question should be interpreted as: "what do LIFTING shackles do?" -- then the answer is they push one end of the spring further from the chassis, thus lifting the truck a little. You get about 1" of lift for every 2" of extra shackle length, in general.