On most sailboats with a furling jib, the jib sheet comes back to a moveable block mounted on the deck, as in this photo. This block can be moved forward or aft for optimum sail shape with different amounts of sail unfurled.
Moving the block forward pulls the clew downward more than back, tightening the sail's leech more than the foot. Moving the block aft pulls the clew back more than down, tightening the sail's foot more than the leach. Find the ideal position by watching the jib telltales at the top and the bottom of the luff in order to have both the top and the bottom of the sail in trim.
Sailors usually mark or note the ideal block position for the sail when fully opened and when partly reefed. It is much easier to move the block when the jibsheet does not have tension on it, while the sail is either furled or on the other tack.
