The SS Yoke Bar is a fantastic tool for squatters with weak upper backs. Rack pulls are great for people that have a tough time locking a deadlift out. More common than not these two issues are related. Check it out as Marshall "Freakshow" Johnson performs a set of rack pulls with a SS Yoke Bar on his back. These are great for pretty much all things posterior chain!
Basically as you will see Marshall do, is pick up the bar as you would with a rack pull and as you come up, set the SS Yoke bar on your back. Take a few steps out, and get ready for the ride.