The aft lowers are the easiest stay to set. They are the loosest stay on the rig compared to the others and take the least load. They attach just below the spreader and come down to the deck. Their purpose is to keep the mast in column as the boat heels over when the heads of the sails pull on the top of the mast.
If they are too loose, the mast will bend to leeward when sailing. This is part of the rigging is adjusted during the test sail.
For the dockside rig tune, you will tighten the stays until they are a bit looser than the forward lowers while making sure the mast stays in column. If there is any slight bow or bend in the mast, you will simply adjust that out using these stays.
If the mast bends to port, tighten the starboard aft lower a bit more.
If the mast bends to starboard, tighten the port aft lower a bit more.
That's all there is to that stay! It's not as complicated as it looks!