There is a tube going from the front of the mast and back to the mast track. In the tube there are two sheaves so the halyard can run inside the mast track up to the tube, over the sheave, through the tube, and then over the next sheave and out on the front of the mast. The tube is glued water tight to the mast, it was rather tricky to glue inside the mast track...

/hakan