Hi Mickael,
If you haven't seen the side by side comparison page >
www.halfbikes.com/compare
The main difference in braking is that the HB2's brakes were used only to slow down the speed, while the HB3's new drum brake can stop almost immediately.
It's actually really easy to get used to the front brake and just after a few tries it becomes quite intuitive. As you're breaking you'll just need to shift your weight - Tipping over is not possible as the center of gravity is well behind the front wheel.