Generally speaking, beginning skaters use block brakes and experienced skaters don’t use brakes at all, preferring to t-stop or just run off their momentum. Alternatives have been tried, including a number of dynamic braking systems triggered by leg placement back in the late-1990s, but they all failed in the market because the benefits of being able to brake with both feet on the ground, side-by-side were outweighed by the additional complexity and expense.
Enter hydraulic brakes for inline skates, which are far more complex and expensive than those earlier experiements ever were.
I admire the home inventor who tinkers on a project until he solves it. It’s a great tradition. But the boffin behind this project has gotten ahead of himself if he thinks it’s worth anywhere near the €335 he’s charging. That’s more than a pair of basic skates costs. And worse, it works with only four models of skates (Rollerblade Speedmachine, Rollerblade Tempest, K2 Radicalskates, and Fila skates with 100mm or 110mm wheels). Of these the Speedmachines are no longer being sold and the Tempest was recalled for risk of mechanical failure.
And a final puzzle about these brakes — the manufacturer posted a video on youtube showing them in use, but the video never shows a person slowing down or stopping!