The flat sided end of the bolt is splined so it digs into the knuckle when its tightened down,this wont turn and you wont need to hold it while removing the nut.The 6 sided nut end is where you impact wrench it off.Then,you need to take an impact (zip) gun with a straight shank "punch" bit to vibrate the threaded end back into the knuckle,pushing out the splined flat headed side.Eventually it will pop out and fly onto the ground.You shouldnt re use these bolts/nuts.They are available at your local GM dealer and worth replacing.When putting back in,orient them the same way they came out (the head sides)and hammer in the flat headed end so the splines catch.When you air gun the nut back in,it will draw the other end in and wont spin around.