The 27 guys can prolly answer this better, but I will try.
The cap shrouds are the standing rigging that holds up the mast. They are tensioned with hi-field levers and on the 27 there is a block & tackle arrangement that gives you your rig tension. Most 27's are non rotating rigs. When I have sails 27's the cap shroud tension was set by sailing to weather taking the slack out of the lee ward, tacking doing the same, making sure the rig is not falling off to leeward.
27's came originally with a single spreade rig. There were some problems with that and many people upgraded to a double spreader rig, a fairly simple retrofit.
