It basically works by having parking stations instead of a "regular" stacker before the pickup station. Said pickup station then calculates how many trains it can fill and enables as many parking stations. The parking stations are also disabled when a train enters it in order to stop trains pathing to it.
The schedule for the trains looks like this:
- Dropoff until inventory empty
- Waiting until 1 second passed
- Pickup until inventory full
So I'm wondering if there is any way to make the trains use the bypass lane and skip the pickup station if the waiting stations are disabled as MiniBetrayal said would happen in his ~2 years old video.