The line between the two halves. sensor: true is a field the scene writes down, so a saved level keeps its checkpoints and pickups. Hearing one is not: a contact is an event, not a property of an object, so it comes through the adapter, and onEnter hands over the other object rather than only saying that something happened.
The gate flashes and adds a tally mark for each crate that falls through it, stopping none of them. Click to drop another.