On a paddleboard, tension shows up straight away

The first time, nearly everyone does the same thing. The fingers close around the rails and the shoulders shoot up towards the ears. The eyes, meanwhile, drop straight down to the feet. Then the board begins to wobble, on a morning when the sea is completely flat.

I teach these classes off Pointe Rouge, early in the morning, before the beach fills up and before Marseille starts sounding its horns. It took me years to work out why I care about it so much. The board reads whatever is going on inside the person standing on it and hands it back within seconds: it does not judge, it translates. Nothing I run in a studio moves that fast.

You cannot fake it for long on water

In a room, with a mat and a floor that stays put, you can compensate for years: squeeze the thighs, clamp the jaw, hold on through effort and call the result technique. I did exactly that for a long time without noticing. On the water it lasts about thirty seconds. Tension turns into a tremor, the tremor turns into a wobble, and sooner or later your palms end up flat on the board. I give the same three cues every session: loosen your hands, lengthen the out-breath, look a long way ahead. With your eyes glued to your feet you are hunting for balance in ten square centimetres; with your eyes on the sea wall you borrow the steadiness of something enormous that never moves. The long exhale handles the rest, dropping the shoulders and taking the edge off the small panic that has just begun.

Nobody has ever won an arm wrestle with a paddleboard.

What actually happens during the hour

1. 01We meet with our feet in the sand and breathe for five minutes before anyone touches a board. I talk through the whole hour in order, so that nobody heads out on the water wondering what is coming.
2. 02We paddle out to a sheltered patch, a few dozen metres from the shore. That crossing counts as the warm-up.
3. 03We anchor the boards and start on our knees, working with the breath, until the slap of small waves under the shins stops feeling strange.
4. 04We build up in stages: all fours, downward dog, then the first standing postures with the feet wide and the knees soft. I hold each posture for a shorter stretch than I would on land and come back to it more often, because out there the mind tires ahead of the legs.
5. 05We finish lying flat, one hand on the belly. That is the bit people bring up weeks later: the sun, the water right under the shoulder blades, nothing left to hold on to.
6. 06We paddle back in no hurry, then sit on the sand for two minutes to notice what has shifted in the body.

Plenty of people turn up convinced you have to be athletic to get up there. The most comfortable ones are almost never the fittest: a woman of sixty who has swum all her life often has an easier hour than a thirty-year-old runner trained to push through everything. Then there is the quiet. Three hundred metres out, no music, no notification, and several people have told me it was their first unreachable hour in months. I do not strap a speaker to my board and I do not take photographs during class. Wellbeing that ends in a story bores me.

Falling in, and what comes after

Almost everyone arrives with that question, asked with a laugh to keep it light. My answer never changes: yes, you might go in, and honestly it does not matter at all. We are in the sea, in summer, next to an object that floats remarkably well. The people who tip in during the first ten minutes relax for the entire rest of the class, and I have seen it too often to call it a coincidence. The expensive part is spending forty minutes not falling. You already know that expense on dry land: the Tuesday in November when you tense up over breakfast about a three o'clock meeting, in the windowless room on the second floor.

• The long exhale when tension climbs. It works in an office as well as it works at sea, and nobody around you notices.
• The habit of lifting your eyes. Staring at whatever sits ten centimetres away is enough to lose your balance, on a board and in an overloaded day alike.
• A tolerance for movement: things move, they will carry on moving, and there is nothing in that to repair.
• A calmer relationship with getting things wrong, for the plain reason that you went in, climbed back on, and nothing happened.

I have no idea whether it works for everyone. Some people come back to the beach with their shoulders down and their voice settled, telling me they have not thought about their day for an hour. Others mostly get cold. In September we set off at first light, the water is still warm, and the sand sticks to wet feet all the way to the car. Bring a second towel.