There are a few different types of adjustable lines, some with plastic buckles and a finger loop for release, and others with metal cleats. Basically, lines are lengthened when rounding the upwind mark, usually just before heading downwind. It's easier to let go with the back hand and release the line if you are slowing down to round or if you are about to tack. If you have to gybe on the downwind run, then you either have to lengthen the other line just after gybing or stay with the short line until you hit the mark. Depends on how much wind and how far you have to go to the mark.
I usually shorten the line just before rounding the downwind mark if alone (no traffic) by sheeting out and slowing up a bit. If crowded, then I will do it after rounding the mark and begin heading back upwind.
Since I am not a great formula racer, I frequently go into the chicken strap when heading downwind when the wind is over 20 knots. If using the chicken strap, then short lines work best and no adjustment has to be made when rounding the mark. However, it's rare for a top sailor to use a chicken strap and some don't have them.
A few formula racers have two sets of harness lines on their booms, one short and one long. They just hook on to the one they need. I once saw a single harness line with two loops. Same deal as two lines, but you only have one to attach to the boom.
I am not a great formula sailor and others like COACHG may have some other suggestions.
I don't recall any top racers holding on to an uphaul line while racing Formula. Many don't even take an uphaul when racing. Standard formula technique means you are overpowered upwind and you better have both hands on the boom. I agree with Coachg on the issue with long harness lines upwind. Even with short lines in strong winds, I have been slammed by waves while heading upwind a few times in Corpus Christi Bay. You can't lean back much in 1 meter chop.
Phil McGain is the one that once told me to keep the board as flat as possible heading upwind, but when fully powered, it does rail up a little. I have watched a lot of top formula sailors ahead of me, going faster and pointing higher with their boards railed no more than 15 degrees.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.