You book your workout in advance in the Workit app, before you go to the sports location. The app shows all partner locations and their classes; you choose a class or open gym and confirm your booking. The sports location receives the booking automatically.
Booking also applies to open gym, except at walk-in partners (see How does a walk-in and the QR code work?). Only book for times you're really going to work out, as the location keeps a spot free for you.