i have seen this on a lot of performance cars such as other novas and sum camaro's what is it?

http://i8.photobucket.com/albums/a22/dahlisnice/whatsthis.jpg

Are you reffering to the traction bars?

The shackle is the 2 bars hanging down from the eye of the leaf spring. They are not the right length for the cars suspension and are hanging down. In the early 70's kids jacked up the back end of the car this way.

Jay's got it if you are talking about the yellow parts...

what does it improve?

They are Slapper bars. They help improve wheel hop on take off.