It had a central so called "REST API" that was deployed on your server and was serving all changes directly to your clients. This lowered traffic and made it robust.
This was the main client for LegionBoard and always supported every feature of Heart.
Thanks to Vertretungsplan.me the changes plan of LegionBoard was able to be displayed nicely even on mobile devices and the users got notified on changes.
You can always look at the code of LegionBoard. It is open source with repositories on GitLab and GitHub.
LegionBoard was based on the newest technologies. Many big companies were using the same stuff.
All the changes can be viewed on any device with low traffic usage.
You can get the source code and deploy it yourself. There are no costs for you. Promised.
Compared to other solutions like PDF schedules that have to be downloaded each time, LegionBoard was very resource efficient and didn't need much traffic.
LegionBoard was developed with security in mind. Thanks to being open source, this can be reviewed every time.