A system like this makes moving the sheet car to the right place a breeze. For the full genoa, just move it fore or aft until any tell-tales at the top and bottom of the luff ‘lift’ at the same time when you’re a few degrees above closehauled. If the top ones ‘break’ first, move it forward, if the bottom ones go first, shunt it aft. When the sail is reefed you probably won’t have any tell-tales so you’ll have to do it by eye alone. It will almost certainly need sliding forward. It’s easy to say, ‘Oh, I won’t bother with all that. I’m not a racing person.’ But a quarter-knot is a quarter-knot, and it’s six miles in twenty-four. A whole hour freezing at sea when you could have been snug in the pub!