Rear is a big adjustment. I would be very careful there . Front is a more common adjustment back for more turning power and shorter tracks forward on the bigger high bite stuff .
Rear lag - two ways too look at it .
One it skews the axle back creating easier rotation .
Two it sets the front end over to the left .
Either way it creates yaw .
better exit, could cause push on entry to apex.
Also lead and lag is a front end only term, the rear is and offset adjustment, it changes the path the rear tires versus the front