The tops of the shrouds are wrapped around thimbles which are then shackled to the thimble on the strop at the top of the topmast.  In actual practice the shrouds were wrapped directly around the top of the topmast.  The shackles have been introduced to enable the rigging to be relatively easily detached from the topmast in the event that the topmast needs to be separated from the lower mast.