The pocket is the part of a wave just ahead of the breaking section, where the face is steepest and the whole thing is trying to throw you.
Stay in it and you are surfing. Drift high and you are standing on a moving hill. Drift low and it closes over you.
That is a game. It has always been a game. It just needed to be about eighty lines of maths.
How it plays
Hold to drop down the face. Release to climb. That is the entire control scheme, and it is one input on purpose, because one input is the difference between a thing you play on a phone at a bus stop and a thing you never open twice.
Your score is seconds spent in the pocket, which is drawn as a red band that travels with the wave. Get more than about twenty and you are doing well. Get more than thirty and I would like to know.
Wipe out by leaving the water, hitting the sky, or getting more than a certain distance from the face. Then paddle back out.
Where the difficulty comes from
Nowhere, deliberately. There is no level design and no difficulty curve.
The wave surface is two sine waves at different wavelengths added together. That is all. Because their wavelengths do not divide into each other neatly, the combined shape never quite repeats, so you cannot learn a rhythm and ride it out. Every few seconds the face does something you have not practised.
I like that a great deal. It is the closest a piece of arithmetic gets to the actual experience of a wave doing something you were not expecting.
The bits that took the longest
The physics took an afternoon. Making it not annoying on a phone took longer.
Holding your finger on a canvas will, by default, scroll the page — so a game about holding becomes a game about the page sliding around. Releasing your finger outside the canvas will, by default, leave the game convinced you are still holding. Leaving the tab open will, by default, keep an animation frame running for the rest of the day.
None of that is interesting to play. All of it is the difference between a toy and something you would send to a friend.
Nothing is kept
Your best score lasts as long as the page does. There is no leaderboard, no account and no saved state, which does mean that the moment you refresh, your 31.4 seconds are gone and nobody will ever believe you.
Take a screenshot. That has always been how surfing evidence works.