Learn harness riding: set lines and boom correctly, hook in and out safely and carry the sail's pull through your hips instead of your arms.
What does the harness do in windsurfing? It transfers the sail's pull via a hook at your hips to your centre of gravity, so your body weight holds you instead of your arms [1]. In the harness you can sail longer in stronger wind without burning forearms.
The honest answer about difficulty: the first time, the harness feels like hanging from a rope above an abyss. After a few runs you feel the difference. That arms and shoulders work hard in windsurfing is shown by a measurement on the water: when beating and reaching (both with pumping) the trapezius muscle (not the harness), forearm flexors and extensors and the biceps were very active in six windsurfers in light to moderate wind [5]. The grip on the boom matters too: in a simulator with six experienced windsurfers, the grip with both hands in supination (palms up) on a 28 mm boom was the most economical for the forearm muscles [6].
In the VDWS system harness riding belongs to level 3 together with the beach start – after the basic licence and fast tack and gybe [7].
Point the sail into the wind, sheet in fully and hang into the sail. The sail's centre of pressure then lies roughly between your hands – the middle of the lines belongs there too. For starters, put the two Velcro straps one to two hand-widths apart [1].
Length check: if the lines reach from wrist to elbow, it usually fits. The boom should not sit below shoulder height. Standing on the board with the sail sheeted in, the lines should end just above the hook [1].
Turn the front foot and shoulders in the direction of travel. Then the force acts forward and the board starts planing earlier [1]. If you sail with a boom that is too low and lines that are too short, your force goes across the board: with feet and shoulders turned to leeward you choke off your drive [1].
On a beam reach, sheet in strongly. Pull the boom toward you and push your hips forward; the shoulders stay back so your centre of gravity does not pull you to leeward. Shift the sail's pull slowly into the harness and relax your arms. Once hooked in you should be able to straighten your arms completely [1].
Milestone: you hang in the harness, arms relaxed, and can hold your course without burning forearms.
Steer with body weight and fine sail movements (easing and sheeting in), not with frantic arm movements. The principle of luffing and bearing away stays the same as without a harness; the only new thing is that your weight holds the pull (practical advice, no study behind it).
Before every tack, gybe or large steering movement, hook out: pull the boom toward you, push your hips forward, keep your shoulders back. The line then slips out of the hook [1].
Before you hook in on the water, try hanging in the harness on land: sail into the wind, sheet in fully, hang. If the lines are too short or too far apart, you notice within seconds. Your head learns: hips forward, shoulders back, arms long.
When you control course, easing and sheeting securely and the sail's pull becomes noticeable in your arms. In the VDWS system the harness is in level 3, after the basic licence and the fast tack and gybe [7].
For starters: if the lines reach from wrist to elbow, it usually fits. Adjustable lines of 22 to 30 inches, about 56 to 76 centimetres, are recommended [1].
On a beam reach, sheet in, pull the boom toward you, push your hips forward and keep your shoulders back. Then shift the sail's pull slowly into the harness and relax your arms [1].
Back complaints are common in windsurfing, but whether the harness is the trigger is not shown by the available studies [2][3]. An overview lists strength training for upper and lower body plus warming up as proposed, only weakly supported prevention [4].
Getting Planing and Into the Footstraps: How to Start Planing