Hi Gaby,
please don’t worry, it is just a trick!
Please check out Odd cyclists video, be very attentive around the time stamp 0:18.
It is just a quick and short push (or pull) either on the right or the left end of the steering bars.
Principially you push the tyres against each other and that helps to open the magnet.
https://www.youtube.com/watch?v=4RbP0elWy7U
One hint more; there is a tiny hex worm screw inside the magnet’s counterpart.
If you turn this screw a litle bit it creates a gap between magnet and the counterpart.
This way you can “adjust” the magnet’s force, the matching Allen key should be a part of the Strida board tools which were usually delivered with your bike 
It’s as easy as riding a bike - once you managed to balance 