What you describe is highly doubtful.. and if true is rigged incorrectly. Very much so.
Far more likely that your furler/jib halyard goes down the inside of the mast, and another adjustable line is your topping lift. I'd be amazed if in fact the same line goes through the masthead sheaves and down to the boom. Pictures would help here....
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"