When a rider passes - press timer button to create an timestamp.
Make sure the timestamp is active (blue, not grey). Now check the rider number. Press their number.
The timestamp should be "assigned" that number and change in appearance. Either "Rider finished" or "Rider passed" (if there are more than one lap).
You can long press an assigned event to unassign it again.
Once all riders have finished or DNF/DNS (long press number to do this) you can go to the result view.