The inboard end of the pole slides on this endless shockcord "trolley" that slopes up and over the mast bail. The shockcord tension has to be just right. If it is too tight it interferes with the mainsail in light winds. If it is too slack the end of the pole droops too much and hits the skipper on the head when tacking. It also has to be terminated at just the right point as shown.
I tried a small swivel block instead of the shackle shown but it kept getting twisted out of line. The small shackle works much better but you must use a smooth forged type so that the shockcord runs smoothly. The black foam sleeve simply stops the pole from clattering against the boom.